Poll the audio timestamp more frequently at the start of playback

Add logic to poll AudioTimestamp at different rates depending on when
playback starts and how the audio timestamp advances.

This fixes a pause about 500 ms after starting playback that occurs on some
devices and also makes our polling interval match the recommendations of the
AudioTrack documentation.

Issue: #3830
Issue: #3841

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189765200

RELEASENOTES.md

@@ -24,10 +24,14 @@
   * Allow adding and removing `MediaSourceEventListener`s to MediaSources after
     they have been created. Listening to events is now supported for all
     media sources including composite sources.
-* Audio: Factor out `AudioTrack` position tracking from `DefaultAudioSink`.
+* Audio:
+  * Factor out `AudioTrack` position tracking from `DefaultAudioSink`.
+  * Fix an issue where the playback position would pause just after playback
+    begins, and poll the audio timestamp less frequently once it starts
+    advancing ([#3841](https://github.com/google/ExoPlayer/issues/3841)).
 * Caching:
   * Add release method to Cache interface.
-  * Prevent multiple instances of SimpleCache in the same folder. 
+  * Prevent multiple instances of SimpleCache in the same folder.
     Previous instance must be released.
 
 ### 2.7.1 ###