frameworks/av: Syncronize resume and onPauseTimeOut

- All the mixer controls are closed 3s after pausing playback
through the onPauseTimeOut function
- A scenario where onPauseTimeOut is closing mixer controls, a
resume is issued, causes a crash
- Synchronize these 2 functions using a mutex to prevent
concurrent execution.

Change-Id: I850452f4d38d05cfa92ca4690a665b8e64170a49
CRs-Fixed: 398948