Revert "Revert "SoundPool: Use caller thread for sound playback""

This reverts commit af018345.

Reason for revert: Further testing indicates there is no regression from the original state before October 23. See bug for more details on testing performed.

Change-Id: Idba0a01621243ab17e9ef607aec75af2c8db2f25
Test: forrest test on builds 5960785 and 5960803 perfetto_system#ion-avg
Bug: 145834064