SDK / exoplayer

- Correctly null out streams[j] in the case that a renderer
  is being disabled.
- Read discontinuities from all children, not just enabled
  ones. This fixes a failure when reading a discontinuity
  with all renderers disabled.
- Add in some assertions to make incorrect stream selection
  failures obvious and immediate.
- Relocate subtitles so they're above the shutter (needed so
  they continue to be visible when video is disabled but
  text is still enabled).

Issue: #1854

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=135089944