title: ExoPlayer 2 Playlist Listener
body: <p>I'm using the new features from ExoPlayer 2.x to play a list of audio files like this:</p>
  
  <pre><code>List&lt;MediaSource&gt; playlist = new ArrayList&lt;&gt;();
  
  ...
  
  ConcatenatingMediaSource concatenatedSource = new ConcatenatingMediaSource(
              playlist.toArray(new MediaSource[playlist.size()]));
  
  mExoPlayer.prepare(concatenatedSource);
  mExoPlayer.setPlayWhenReady(true);
  </code></pre>
  
  <p>This is working fine, but in order to update my UI accordingly, I need to know which track is currently playing and the progress of this track. Is there any listener from ExoPlayer?</p>
  
  <p>Thanks!</p>