SDK / exoplayer

Move initialization code from the audio renderers into handleBuffer. The audio
session id and tunneling enabled state can be altered using new setters.

This change is not intended to change the behavior of the audio renderers.
Before this change, the AudioTrack would always be reinitialized by the
renderers before calling handleBuffer.

Add a protected onAudioTrackUnderrun method for custom renderers that want to
implement custom handling of underruns.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=144067149