SDK / exoplayer

- Made enabledRenderers an array to avoid loads of method calls.
- Made if so that enabled renderers are always called in a consistent
  order, rather than their order changing if they're enabled/disabled
  over time. This is likely to make performance more predictable.
- Split out reading of resets into a separate method. This method is
  now called directly after seeking on the source, so as to ensure
  instant propagation of the new position from source->renderers in
  the common case.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=117225639