SDK / exoplayer

- Remove getNewPlayingPeriodIndex from MediaSource
- Remove all absolute references to period indices from
  Window. Their existence prevents ConcatenatingMediaSource
  from being able to efficiently handle children with
  large numbers of entries (e.g. a cursor backed source),
  since it would copy all windows into its merged timeline.
- Fix ExoPlayerImplInternal to attempt a restart in the
  case that the loading (but not yet prepared) period is
  removed, in absence of a playing period.
- Implement logic for finding the "next" period in the old
  timeline when attempting a retry.

Removing some of the nasty US<->MS conversions left as a
virtual TODO.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=130659973