title: Stop animation when pause button clicked in Exoplayer Notification
body: <p>my problem is that I added an Exoplayer Notification to my app, and I want when I press the "Pause" button, to stop an animation from the MainActivity. I have an animation to an ImageButton. I know how to stop the animation under normal circumstances,
  but I do not know how stop the animation from the Notification. Below I'll add you my notification code.
  Thank you in advance !!!</p>
  
  <pre><code>notificationManager = PlayerNotificationManager.createWithNotificationChannel(
              this,
              CHANNEL_ID,
              R.string.exo_download_notification_channel_name,
              NOTIFICATION_ID,
              new PlayerNotificationManager.MediaDescriptionAdapter() {
                  @Override
                  public String getCurrentContentTitle(Player player) {
                      return text_song;
                  }
  
                  @Nullable
                  @Override
                  public PendingIntent createCurrentContentIntent(Player player) {
                      Intent intent = new Intent(MainActivity.this, MainActivity.class);
                      PendingIntent contentPendingIntent = PendingIntent.getActivity
                              (MainActivity.this, 0, intent, 0);
                      return contentPendingIntent;
                  }
                  @Nullable
                  @Override
                  public String getCurrentContentText(Player player) {
                      return null; 
                  }
  
                  @Nullable
                  @Override
                  public Bitmap getCurrentLargeIcon(Player player, PlayerNotificationManager.BitmapCallback callback) {
  
                      return null;
                  }
              }
      );
      notificationManager.setUseChronometer(true);
      notificationManager.setUseNavigationActions(false);
      notificationManager.setFastForwardIncrementMs(0);
      notificationManager.setRewindIncrementMs(0);
      notificationManager.setPriority(NotificationCompat.PRIORITY_HIGH);
      notificationManager.setPlayer(player);
  </code></pre>