Fix where scanner thread is quit.

This fixes what I believe was a simply copy-paste error regarding how
the scanner thread is quit. This uses the scanner thread handler,
instead of the main thread handler, to schedule the `quit` operation.
This way any queued scan jobs (such as a stop) complete before we quit
the thread. This is what the comment states, but wasn't what the code
implemented.

The `mHandler.removeCallbacksAndMessages` is removed from the background
job as `destroy` is already on the main thread. So we want to clear any
other jobs still in the queue to run later now as we are in the process
of destroying everything.

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

@@ -16,6 +16,7 @@
 import android.os.SystemClock;
 import android.support.annotation.MainThread;
 import android.support.annotation.NonNull;
+import android.support.annotation.WorkerThread;
 
 import org.altbeacon.beacon.BeaconManager;
 import org.altbeacon.beacon.logging.LogManager;
@@ -198,17 +199,19 @@ public void setDistinctPacketsDetectedPerScan(boolean detected) {
     @MainThread
     public void destroy() {
         LogManager.d(TAG, "Destroying");
+
+        // Remove any postDelayed Runnables queued for the next scan cycle
+        mHandler.removeCallbacksAndMessages(null);
+
         // We cannot quit the thread used by the handler until queued Runnables have been processed,
         // because the handler is what stops scanning, and we do not want scanning left on.
         // So we stop the thread using the handler, so we make sure it happens after all other
         // waiting Runnables are finished.
-        mHandler.post(new Runnable() {
-            @MainThread
+        mScanHandler.post(new Runnable() {
+            @WorkerThread
             @Override
             public void run() {
                 LogManager.d(TAG, "Quitting scan thread");
-                // Remove any postDelayed Runnables queued for the next scan cycle
-                mHandler.removeCallbacksAndMessages(null);
                 mScanThread.quit();
             }
         });