SDK / exoplayer

This is equivalent to DashTrackSelector and SmoothStreamingTrackSelector.
This is a step toward allowing HlsChunkSource to expose multiple tracks,
which is a requirement for supporting WebVtt.

This change also enables WebVtt extractor instantiation.

Issue: #151
Issue: #676
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111698833