SDK / exoplayer

This adds options to ExoPlayer.sendMessages which allow to specify a window index
and position at which the message should be sent. Additionally, the options can be
configured to use a custom Handler for the messages and whether the message should
be repeated when playback reaches the same position again.

The internal player converts these window positions to period index and position
at the earliest possibility. The internal player also attempts to update these
when the source info is refreshed. A sorted list of pending posts is kept and the
player triggers these posts when the playback position moves over the specified
position.

Issue:#2189

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179683841