Merged media in a playlist randomly stops playing unless manually seek
[bugreport.txt](https://github.com/google/ExoPlayer/files/740784/bugreport.txt)
So in my application, user can select different languages. Based on user preference I use different audio files to be merged with video files. Now, once the playlist starts playing the merged tracks, it randomly stops playing if the seekbar stops "buffering". I then have to manually touch the seekbar so it starts playing again. Note that this doesn't happen if I only use the video files without merging them. My files are all local (mp4 videos and mp3 audios) and stored on the device. The duration of the files are the same but I'm not sure if this has to be accurate to milliseconds. 

This is how I setup the player:
```java
// 1. Create a default TrackSelector
Handler mainHandler = new Handler();
BandwidthMeter bandwidthMeter = new DefaultBandwidthMeter();
TrackSelection.Factory videoTrackSelectionFactory = new AdaptiveVideoTrackSelection.Factory(bandwidthMeter);
TrackSelector trackSelector = new DefaultTrackSelector(videoTrackSelectionFactory);
// 2. Create a default LoadControl
LoadControl loadControl = new DefaultLoadControl();
// 3. Create the player
mPlayer = ExoPlayerFactory.newSimpleInstance(this, trackSelector, loadControl);
// 4. Bind the player to View
mExoPlayerView.setPlayer(mPlayer);
// 5. Prepare the player
mDataSourceFactory = new DefaultDataSourceFactory(this, Util.getUserAgent(this, "My APP"), null);
mExtractorsFactory = new DefaultExtractorsFactory();
```

Here's the code for starting the player:
```java
ArrayList<MediaSource> mediaSources = generateListVideos();
ConcatenatingMediaSource concatenatedSource = new ConcatenatingMediaSource(mediaSources.toArray(new MediaSource[mediaSources.size()]));
mPlayer.addListener(this);
mPlayer.prepare(concatenatedSource, true, true);
mPlayer.setPlayWhenReady(true);
```
I'm not if the problem is my media files or it's the use of `MergingMediaSource` that causes this issue. Any help would be appreciated.

I am using version 2.1.1 and I have seen the same issues on a Galaxy note 3 (4.3) and a LG G5 (7.0). 
