SDK / exoplayer

This is the first step toward merging of RollingSampleBuffer
and DefaultTrackOutput, which is a precursor to removing some
indirection for DASH/SS playbacks, and to moving HLS playbacks
to use a single output buffer per track (with inline splicing).

Where this is heading is that sample format changes will
eventually be attached to samples in the rolling buffer. This
will eliminate the need for piping sample formats around the
edges of the rolling buffer (e.g. via redirection in
ChunkExtractorWrapper and BaseMediaChunk.getSampleFormat).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120123093