DefaultAudioSink had a lot of code related to tracking the position of the
underlying AudioTrack, and working around issues with the position APIs.
Move this code to a separate class responsible for position tracking. This is in
preparation for improving the audio timestamp polling behavior.
This change is intended not to cause any behavior changes.
Issue: #3841
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189344743
