SDK / exoplayer

This requires knowing the seek time in Extractor.seek, so that it's possible to
pick the latest synchronization sample at/before the seek time for each track
(rather than the earliest synchronization sample after the seek position).

Also remove the STATE_AFTER_SEEK state which should no longer be needed.

Issue: #2167

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=141432598