Future proof threading / scheduled task flows

This adds a guard to ensure that we only attempt a further scan when we
the scan cycler is started. This helps ensure we've terminated scanning
for the case where all regions have been removed but the service is
still bound. In this case only `stop` will be called (instead of both
`stop` and `destroy`). When this happens there are two potential
scheduled tasks still queued:

- `scanLeDevice(true)` from `deferScanIfNeeded`
- `scheduleScanCycleStop`

Which potential task is queued is based on if we were in an active scan
period or a between scan period when `stop` was called. In the case of a
between scan period the `deferScanIfNeeded` _will_ cause scanning to
start up again when it ends. It will run for one more scan cycle before
it terminates in `finishScanCycle`. Similarly, if we are in an active
scan period the scheduled job for finishing the scan cycle will continue
for the duration of the scan period.

CHANGELOG.md

@@ -10,6 +10,8 @@ Bug Fixes:
 
 - Fix failure to stop scanning when unbinding from service or when the between scan period
   is nonzero. (#507, David G. Young)
+- Fix failure to stop scanning when there are no active ranged or monitored
+  regions. (#513, Aaron Kromer)
 
 ### 2.10 / 2017-04-21
 

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

@@ -230,7 +230,7 @@ protected void scanLeDevice(final Boolean enable) {
             if (getBluetoothAdapter() == null) {
                 LogManager.e(TAG, "No Bluetooth adapter.  beaconService cannot scan.");
             }
-            if (enable) {
+            if (enable && mScanningEnabled) {
                 if (deferScanIfNeeded()) {
                     return;
                 }
@@ -294,7 +294,7 @@ protected void scanLeDevice(final Boolean enable) {
     protected void scheduleScanCycleStop() {
         // Stops scanning after a pre-defined scan period.
         long millisecondsUntilStop = mScanCycleStopTime - SystemClock.elapsedRealtime();
-        if (millisecondsUntilStop > 0) {
+        if (mScanningEnabled && millisecondsUntilStop > 0) {
             LogManager.d(TAG, "Waiting to stop scan cycle for another %s milliseconds",
                     millisecondsUntilStop);
             if (mBackgroundFlag) {