Unregister sharedpreferences listener on destroy.

If the playback service is stopped before the main activity gets killed, this would have caused the old class to still get updates.

Sending preference updates to such an orphaned PlaybackService would trigger crashes in some cases (especially when MediaLibrary detects suspicious unregisters).

app/src/main/java/ch/blinkenlights/android/vanilla/PlaybackService.java

@@ -624,6 +624,10 @@ public final class PlaybackService extends Service
 		// defer wakelock and close audioFX
 		enterSleepState();
 
+		// stop getting preference changes.
+		getSettings(this).unregisterOnSharedPreferenceChangeListener(this);
+
+		// shutdown all observers.
 		MediaLibrary.unregisterLibraryObserver(mObserver);
 		mPlaylistObserver.unregister();