SDK / exoplayer

When a seek is performed, renderers currently perform the
actions that they need to take in two places: Some changes
are performed in seekTo implementations. Other changes are
performed when discontinuities are read from the source.

In HLS we need to perform what is effectively a seek
originating in the source. To support this, this CL allows
discontinuities read from the source to modify the playback
position. All actions that renderers perform as a result
of a seek are moved to be performed when a discontinuity is
received.

Best way to understand CL:
- Look at SampleSource interface change and then at the
  concrete implementations, to make sure they've been
  changed properly.
- Look at SampleSourceTrackRenderer change.
- Look at concrete renderers. The general pattern is that
  code previously performed in seekTo and READ_DISCONTINUITY
  is merged into onDiscontinuity().

Note: This will be further untangled in V2.

Issue #676
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112720746