- This is needed to make DefaultTrackSelector properly thread
safe, and enable atomic changes to the selector that, for
example, disable a renderer and enable another one at the same
time.
- This change also saves/restores parameters and the start
position in PlayerActivity, resolving the ref'd issue.
Issue: #3915
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193913350
