title: Android ExoPlayer onProgressChanged
body: <p>How can I monitor progress changes on <code>ExoPlayer</code>?<br>
  I tried to implement a hidden <code>MediaController</code> and overriding <code>setOnSeekBarChangeListener</code> methods, but for now without success. I'm wondering if there is another way to listen to the <code>ExoPlayer</code> progress.</p>