This change restores ExoPlayer to its previous behaviour.
I think we'll want a "resetPosition" boolean parameter on
setMediaSource at some point, but no need to add it right
now. Note that this would not always reset to the start
of the source. For a live playback it will reset to the
desired position (normally somewhere slightly behind the
live edge).
Issue: #1667
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127549700
