Minimize memory leak risks.

Remove implicit back-reference from playback thread to player.
Use new MediaCodecList APIs on L.
Add support for extracting Vorbis audio in WebM Extractor.
Add missing class.
Add receiver for HDMI plugged-in configuration changes.
Factor out AudioTrack from MediaCodecAudioTrackRenderer.

AudioTrack contains the portions of MediaCodecAudioTrackRenderer that handle the
platform AudioTrack instance, including synchronization (playback position
smoothing), non-blocking writes and releasing.

This refactoring should not affect the behavior of audio playback, and is in
preparation for adding an Ac3PassthroughAudioTrackRenderer that will use the
AudioTrack.