title: Add backbutton to MediaController
body: <p>I have a Video-Activity with a MediaCrontroller like this</p>
  
  <pre><code>    ctlr = new MediaController(this);
      ctlr.setMediaPlayer(mVV);
      mVV.setMediaController(ctlr);
      ctlr.show();
  </code></pre>
  
  <p>how can i add a back Button to the MediaController? (The system navigation bar is not available since it runs on a dedicated device)</p>