SDK / exoplayer

- Make read return 0 if readLength==0
- Return RESULT_END_OF_INPUT properly in the case that
  bytesRemaining is unset (this was broken previously,
  but only applies for assets > 2^31 bytes, so it's
  unlikely anyone ever hit this issue)

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135136541