API level 21 enhancements for ExoPlayer playbacks.

- Use native frame release timing in video renderer for
  smoother video playback.
- Avoid unnecessary memory copy steps in audio renderer.
- Use non-blocking AudioTrack API.
Update CaptionStyleCompat for L.
Merge pull request #63 from google/dev

Update dev-l
Merge pull request #65 from google/dev

Add file missing from "Improve subtitle handling" change.
Fix incorrect condition.
Merge pull request #74 from google/dev

Merge dev -> dev-l
Use setVolume on API level 21 devices, plus minor naming cleanup.
Tweak audio renderer to match dev/dev-hls.
Remove additional "/" from merged URLs.

Issue: #81
Update HlsSampleSource + correctly propagate error from prepare.

Issue: #81
Add case for 7.1 audio.
Make mpd parser more ameanable for extension.
Merge pull request #82 from google/dev

Pull dev -> dev-hls
Merge pull request #83 from google/dev

Pull dev -> dev-l
Parse all UUID boxes, not just the first one.
Let FileDataSource report to a TransferListener.
Add some MimeTypes that will be useful in the future.
Add a UriDataSource for reading from file or network.
Allow direct and indirect buffer replacement.

Also tweak ManifestFetcher.
Cleanup TextTrackRenderer.