title: set duration playing with exoplayer
body: <p>I have a service player, which is working well.
  I have list audio sounds, which I want to play sequentially during the different minute times.</p>
  
  <p>My code : </p>
  
  <pre><code> val sourceList = mutableListOf&lt;ExtractorMediaSource&gt;()
      listSounds.forEach {
         val uri = RawResourceDataSource.buildRawResourceUri(it)
         val source = ExtractorMediaSource(uri, dataSourceFactory, extractorsFactory, null, null)
         sourceList.add(source)
      }
  mPlayer!!.prepare(ConcatenatingMediaSource(*sourceList.toTypedArray()))
  </code></pre>
  
  <p>I want to repeat the source list for 10 or 20 minutes. How can I set the duration ?</p>