SoundPool: Use caller thread for sound playback

A worker playback thread may be starved under high usage situations
such as screenshots or camera snapshots.

Also fix issue with caller thread playback when reusing stream on restart queue.

Test: camera snapshot, screenshot
Test: native soundpool_stress
Test: android.media.cts.SoundPool*Test
Bug: 143448491
Bug: 144540753
Change-Id: I126c3efffc1f1dfb0fcde7a71cdea64afbe0af0a