SDK / exoplayer

- Fixes the referenced issue.
- Provides more consistent behavior for the edge case of empty
  chunks (i.e. chunks containing now samples). Format changes
  will now always be reported for such chunks, unless the empty
  chunk is discarded from upstream before there's a chance to
  do so.

Issue: #4533

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212443043