Factor out position tracking from DefaultAudioSink

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

RELEASENOTES.md

@@ -21,6 +21,7 @@
     deprecated `DynamicConcatenatingMediaSource`.
     deprecated `DynamicConcatenatingMediaSource`.
   * Allow clipping of child media sources where the period and window have a
   * Allow clipping of child media sources where the period and window have a
     non-zero offset with `ClippingMediaSource`.
     non-zero offset with `ClippingMediaSource`.
+* Audio: Factor out `AudioTrack` position tracking from `DefaultAudioSink`.
 
 
 ### 2.7.1 ###
 ### 2.7.1 ###