SDK / exoplayer

When the player state is changed from an event listener callback, we may
get recursive listener notifications. These recursions can produce a wrong
order, skip or duplicate updates, and send different notifications to
different listeners.

This change serializes listener notifications by clustering all update data
in a helper data class and adding the updates to a queue which can be handled
in a loop on the outer layer of the recursion.

As playWhenReady updates also reference the current playbackInfo, we need to
redirect the listener notifcations for setPlayWhenReady to the same queue.

Issue:#4276

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198031431