The playback state in ExoPlayerImpl and ExoPlayerImplInternal is usually kept
in sync. Only the timeline was so far not updated in the same way with the
internal player using a null timeline while ExoPlayerImpl keeps the previous
timeline.
This change removes the need to keep a null timeline which allows to update
the internal timeline in the same way as the external one. This fixes problems
when retrying a failed playback multiple times.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188034988
