convert PlayerHolder to Singleton, handle context within, bugfix ServiceConnection leak

- bugfix: have ServiceConnection created only once!

- select the context within the PlayerHolder to start, stop, bind or unbind the service
  -> we have to make sure the Service is started AND stopped within the same context
  -> so let PlayerHolder be the one to select the context

- remove removeListener() and replace the call with setListener(null)
- Compatibility: use ContextCompat.startForegroundService instead of startService()

app/src/main/java/org/schabi/newpipe/MainActivity.java

@@ -823,7 +823,7 @@ public class MainActivity extends AppCompatActivity {
             return;
         }
 
-        if (PlayerHolder.isPlayerOpen()) {
+        if (PlayerHolder.getInstance().isPlayerOpen()) {
             // if the player is already open, no need for a broadcast receiver
             openMiniPlayerIfMissing();
         } else {