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.
Merge pull request #83 from google/dev

Pull dev -> dev-l
Merge pull request #92 from google/dev

Dev -> Dev-l
Merge pull request #97 from google/dev

dev
Merge pull request #100 from google/dev

dev -> dev-l
Merge pull request #113 from google/dev

dev -> dev-l
Merge pull request #115 from google/dev

Use largeHeap in demo app
Merge pull request #121 from google/dev

dev -> dev-l
Merge pull request #127 from google/dev

dev -> dev-l
Merge pull request #131 from google/dev-l

Merge dev-l into dev-hls, now L SDK is released.