SDK / exoplayer

This is a step toward retaining a back-buffer in a way that
works for all MediaSource implementations. It's not possible
to adjust the discardBuffer calls in ExoPlayerImplInternal
to discard up to (position - backBufferDurationUs). Next steps
are to:

1. Find an appropriate place to specify the back buffer value,
   to be passed to the discardBuffer calls. I guess the
   LoadControl is the appropriate place to define such values.
2. Enhance discardBuffer to support a toKeyframe argument to
   pass through to discardTo.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175565363