Currently, the looper of the thread the player is created on is used (or the
main looper if this thread doesn't have a looper). To allow more control over
the threading, this change lets users specificy the looper which must be used
to call player methods and which is used for event callbacks.
Issue:#4278
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201331564
