Start documenting thread usage for service objects

This makes the current implicit assumption that the scan cycler is
started/stopped on the main thread explicit. This helps quickly inform
developers what the callback expectations are in regards to race
conditions and state safety.

In order to verify this we need to trace the `start`/`stop`/`destroy`
calls back to the service. The service's core methods `onCreate` and
`onDestroy` are called on the main thread (see links below). This means
the service creates it's `mMessenger` on the main thread, which creates
the `IncomingHandler` on the main thread which binds to the main looper.
However, the `IncomingHandler` class itself leaves the looper
unspecified relying on looper for the thread which creates it.

As we did in #430, this modifies `IncomingHandler` to explicitly use the
main looper as a future proofing mechanism. This way we can ensure this
implicit expectation doesn't change or at least make it obvious when
troubleshooting cases where someone expects it to have changed.
Similarly, this adds the main thread annotation to it's `handleMessage`
implementation.

Working from here we can confirm that the only places where the beacon
monitoring/ranging is updated is from the main thread through the
`IncomingHandler`. As the `IncomingHandler` is the main communication
channel for the service we transfer the main thread expectation to the
associated ranging/monitoring methods. _If_ someone decides to call
these methods directly on the service these annotations help the IDEs
check/document that such calls are expected from the main thread.

With all of that documented we can now confidently state that the scan
cycler's `start`, `stop`, and `destroy` are also meant to be called from
the main thread. As `start` and `stop` both call `scanLeDevice` we can
start tracing any other threading expectations for it. We already know
it's called from the main thread through deferred jobs. This leaves the
`finishScanCycle` as the last call site.

`finishScanCycle` is only called from `scheduleScanCycleStop`. As this
method name implies it's called through a deferred job on the main
thread. It is also called the "first" time in `scanLeDevice`. Thus we've
shown that `scanLeDevice`, `finishScanCycle`, and
`scheduleScanCycleStop` are expected to run on the main thread.

See Also:

- https://developer.android.com/reference/android/os/Handler.html#Handler%28%29
- https://developer.android.com/training/multiple-threads/communicate-ui.html
- https://developer.android.com/reference/android/app/Service.html
- https://developer.android.com/guide/components/services.html
- #430

src/main/java/org/altbeacon/beacon/service/BeaconService.java

@@ -37,8 +37,10 @@
 import android.os.Build;
 import android.os.Handler;
 import android.os.IBinder;
+import android.os.Looper;
 import android.os.Message;
 import android.os.Messenger;
+import android.support.annotation.MainThread;
 import android.support.annotation.NonNull;
 
 import org.altbeacon.beacon.Beacon;
@@ -145,9 +147,16 @@ static class IncomingHandler extends Handler {
         private final WeakReference<BeaconService> mService;
 
         IncomingHandler(BeaconService service) {
+            /*
+             * Explicitly state this uses the main thread. Without this we defer to where the
+             * service instance is initialized/created; which is usually the main thread anyways.
+             * But by being explicit we document our code design expectations for where things run.
+             */
+            super(Looper.getMainLooper());
             mService = new WeakReference<BeaconService>(service);
         }
 
+        @MainThread
         @Override
         public void handleMessage(Message msg) {
             BeaconService service = mService.get();
@@ -206,7 +215,7 @@ else if (msg.what == MSG_SYNC_SETTINGS) {
      */
     final Messenger mMessenger = new Messenger(new IncomingHandler(this));
 
-
+    @MainThread
     @Override
     public void onCreate() {
         bluetoothCrashResolver = new BluetoothCrashResolver(this);
@@ -294,6 +303,7 @@ public boolean onUnbind(Intent intent) {
         return false;
     }
 
+    @MainThread
     @Override
     public void onDestroy() {
         LogManager.e(TAG, "onDestroy()");
@@ -330,6 +340,7 @@ private PendingIntent getRestartIntent() {
     /**
      * methods for clients
      */
+    @MainThread
     public void startRangingBeaconsInRegion(Region region, Callback callback) {
         synchronized (rangedRegionState) {
             if (rangedRegionState.containsKey(region)) {
@@ -342,6 +353,7 @@ public void startRangingBeaconsInRegion(Region region, Callback callback) {
         mCycledScanner.start();
     }
 
+    @MainThread
     public void stopRangingBeaconsInRegion(Region region) {
         int rangedRegionCount;
         synchronized (rangedRegionState) {
@@ -355,13 +367,15 @@ public void stopRangingBeaconsInRegion(Region region) {
         }
     }
 
+    @MainThread
     public void startMonitoringBeaconsInRegion(Region region, Callback callback) {
         LogManager.d(TAG, "startMonitoring called");
         monitoringStatus.addRegion(region, callback);
         LogManager.d(TAG, "Currently monitoring %s regions.", monitoringStatus.regionsCount());
         mCycledScanner.start();
     }
 
+    @MainThread
     public void stopMonitoringBeaconsInRegion(Region region) {
         LogManager.d(TAG, "stopMonitoring called");
         monitoringStatus.removeRegion(region);
@@ -371,6 +385,7 @@ public void stopMonitoringBeaconsInRegion(Region region) {
         }
     }
 
+    @MainThread
     public void setScanPeriods(long scanPeriod, long betweenScanPeriod, boolean backgroundFlag) {
         mCycledScanner.setScanPeriods(scanPeriod, betweenScanPeriod, backgroundFlag);
     }

src/main/java/org/altbeacon/beacon/service/scanner/CycledLeScanner.java

@@ -1,7 +1,6 @@
 package org.altbeacon.beacon.service.scanner;
 
 import android.Manifest;
-import android.annotation.SuppressLint;
 import android.annotation.TargetApi;
 import android.app.AlarmManager;
 import android.app.PendingIntent;
@@ -125,6 +124,7 @@ public static CycledLeScanner createScanner(Context context, long scanPeriod, lo
      * between LOW_POWER_MODE vs. LOW_LATENCY_MODE
      * @param backgroundFlag
      */
+    @MainThread
     public void setScanPeriods(long scanPeriod, long betweenScanPeriod, boolean backgroundFlag) {
         LogManager.d(TAG, "Set scan periods called with %s, %s Background mode must have changed.",
                 scanPeriod, betweenScanPeriod);
@@ -165,6 +165,7 @@ public void setScanPeriods(long scanPeriod, long betweenScanPeriod, boolean back
         }
     }
 
+    @MainThread
     public void start() {
         LogManager.d(TAG, "start called");
         mScanningEnabled = true;
@@ -175,7 +176,7 @@ public void start() {
         }
     }
 
-    @SuppressLint("NewApi")
+    @MainThread
     public void stop() {
         LogManager.d(TAG, "stop called");
         mScanningEnabled = false;
@@ -194,6 +195,7 @@ public void setDistinctPacketsDetectedPerScan(boolean detected) {
         mDistinctPacketsDetectedPerScan = detected;
     }
 
+    @MainThread
     public void destroy() {
         LogManager.d(TAG, "Destroying");
         // We cannot quit the thread used by the handler until queued Runnables have been processed,
@@ -218,7 +220,7 @@ public void run() {
 
     protected abstract void startScan();
 
-    @SuppressLint("NewApi")
+    @MainThread
     protected void scanLeDevice(final Boolean enable) {
         try {
             mScanCyclerStarted = true;
@@ -285,6 +287,7 @@ protected void scanLeDevice(final Boolean enable) {
         }
     }
 
+    @MainThread
     protected void scheduleScanCycleStop() {
         // Stops scanning after a pre-defined scan period.
         long millisecondsUntilStop = mScanCycleStopTime - SystemClock.elapsedRealtime();
@@ -308,6 +311,7 @@ public void run() {
 
     protected abstract void finishScan();
 
+    @MainThread
     private void finishScanCycle() {
         LogManager.d(TAG, "Done with scan cycle");
         try {