SDK / exoplayer

- The main goal of this change is to remove the need for
  discardSamplesForDisabledTracks() in continueBuffering
  in HlsTrackStreamWrapper and ExtractorSampleSource. As
  a result we'll also save one Allocation per disabled
  track.
- Another benefit of this change is that Allocator.trim
  calls can no longer be dropped. This could previously
  happen on player release if the playback thread's
  Looper quit before delivery of onLoadCanceled which
  performed the trim in the case that a load needed to be
  canceled at the time of the release.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123957434