Fixed IllegalArgumentException in LocationServicesOkObservableApi23Fa…

…ctory (#573)

Due to a race condition and subscribing/unsubscribing from different threads it may have happened that unregistering was made before registering finished.

rxandroidble/src/main/java/com/polidea/rxandroidble2/internal/util/LocationServicesOkObservableApi23Factory.java

@@ -13,6 +13,7 @@
 import io.reactivex.ObservableEmitter;
 import io.reactivex.ObservableOnSubscribe;
 import io.reactivex.functions.Cancellable;
+import io.reactivex.schedulers.Schedulers;
 
 @TargetApi(Build.VERSION_CODES.KITKAT)
 public class LocationServicesOkObservableApi23Factory {
@@ -30,7 +31,7 @@ public class LocationServicesOkObservableApi23Factory {
     public Observable<Boolean> get() {
         return Observable.create(new ObservableOnSubscribe<Boolean>() {
             @Override
-            public void subscribe(final ObservableEmitter<Boolean> emitter) throws Exception {
+            public void subscribe(final ObservableEmitter<Boolean> emitter) {
                 final boolean initialValue = locationServicesStatus.isLocationProviderOk();
                 final BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
                     @Override
@@ -43,11 +44,14 @@ public void onReceive(Context context, Intent intent) {
                 context.registerReceiver(broadcastReceiver, new IntentFilter(LocationManager.MODE_CHANGED_ACTION));
                 emitter.setCancellable(new Cancellable() {
                     @Override
-                    public void cancel() throws Exception {
+                    public void cancel() {
                         context.unregisterReceiver(broadcastReceiver);
                     }
                 });
             }
-        }).distinctUntilChanged();
+        })
+                .distinctUntilChanged()
+                .subscribeOn(Schedulers.trampoline())
+                .unsubscribeOn(Schedulers.trampoline());
     }
 }