AudioTrack: Improve pause handling.

Add pauseAndWait() method to ensure that the pause is completed
(ramped to silence) before any other operation, especially flush().
Incorporate into MediaPlayer pause().

Test: Clarity video with scrubbing bar.
Test: Ringtone and Alarm playback.
Bug: 196194083
Change-Id: Id8842c6c8abfd76d11b8316a95052c2e0da7fb7c
7 files changed