Support additional passthrough audio formats

I'm having trouble getting Exo (2.0.4) to play mkv files with AC-3 or DTS (DCA) audio. This is on a device that should support it (Nvidia Shield hooked to an AVR with support for those and more).

All my searches have come up with the basic answer that this error indicates an unsupported audio track but, given the above hardware, I don't understand how that could be. Is there some sort of additional configuration necessary to make pass thru of these tracks to the AVR possilble? I've just used the default everything from the example/demo app.

Thanks for any direction...

```
12-02 16:44:33.601 E/AudioTrack: AudioFlinger could not create track, status: -22
12-02 16:44:33.605 E/AudioTrack-JNI: Error -22 initializing AudioTrack
12-02 16:44:33.605 E/android.media.AudioTrack: Error code -20 when initializing AudioTrack.
12-02 16:44:33.620 E/ExoPlayerImplInternal: Renderer error.
com.google.android.exoplayer2.ExoPlaybackException
at com.google.android.exoplayer2.audio.MediaCodecAudioRenderer.processOutputBuffer(MediaCodecAudioRenderer.java:345)
at com.google.android.exoplayer2.mediacodec.MediaCodecRenderer.drainOutputBuffer(MediaCodecRenderer.java:890)
at com.google.android.exoplayer2.mediacodec.MediaCodecRenderer.render(MediaCodecRenderer.java:477)
at com.google.android.exoplayer2.ExoPlayerImplInternal.doSomeWork(ExoPlayerImplInternal.java:431)
at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:265)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:148)
at android.os.HandlerThread.run(HandlerThread.java:61)
at com.google.android.exoplayer2.util.PriorityHandlerThread.run(PriorityHandlerThread.java:40)
Caused by: com.google.android.exoplayer2.audio.AudioTrack$InitializationException: AudioTrack init failed: 0, Config(48000, 252, 49152)
at com.google.android.exoplayer2.audio.AudioTrack.checkAudioTrackInitialized(AudioTrack.java:909)
at com.google.android.exoplayer2.audio.AudioTrack.initialize(AudioTrack.java:461)
at com.google.android.exoplayer2.audio.MediaCodecAudioRenderer.processOutputBuffer(MediaCodecAudioRenderer.java:337)
at com.google.android.exoplayer2.mediacodec.MediaCodecRenderer.drainOutputBuffer(MediaCodecRenderer.java:890) 
at com.google.android.exoplayer2.mediacodec.MediaCodecRenderer.render(MediaCodecRenderer.java:477) 
at com.google.android.exoplayer2.ExoPlayerImplInternal.doSomeWork(ExoPlayerImplInternal.java:431) 
at com.google.android.exoplayer2.ExoPlayerImplInternal.handleMessage(ExoPlayerImplInternal.java:265) 
at android.os.Handler.dispatchMessage(Handler.java:100) 
at android.os.Looper.loop(Looper.java:148) 
at android.os.HandlerThread.run(HandlerThread.java:61) 
at com.google.android.exoplayer2.util.PriorityHandlerThread.run(PriorityHandlerThread.java:40)
```