title: Exoplayer: How to get running Media Source
body: <p>I have an App, that shows pictures and videos from the Internet.
  Now I'm implementing a "save" function, that saves the picture/video to external or internal storage, without downloading them a second time.</p>
  
  <p>For the pictures I just grab the drawable with <code>imageView.getDrawable()</code>.
   How would I achieve the same with the video thats currently buffered in the Exoplayer2.x / PlayerView? I'm not finding a <code>getMediaSource()</code> function.</p>