SDK / exoplayer

- It's a no-op for now
- Renderers that want to support retaining resources will move
  some functionality from their disable() implementations into
  reset()
- ExoPlayerImplInternal will be updated to not always call reset()
  immediately after disable(), which is what will enable resources
  to actually be retained.

Issue: #2826

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215729783