ExoPlayer can be run on a background thread, but some components (UI and IMA)
only support players on the main thread. This adds some documentation and
assertions for that.
To simplify assertions, this also moves the getApplicationLooper method from
ExoPlayer to Player.
Issue:#4439
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213280359
