frameworks/base: Fix for LPA volume sequence reversal

- LPA volume is sometimes reversed while doing volume Up&DOwn.
- Two different threads are trying to set volume, one by invoking
  setSessionVolume() and other by invoking setStreamVolume().
  But currently only setStreamVolume() is mutex protected. This
  is leading to race condition and volume set second is getting
  applied first.
- Fix is to acquire mutex in setSessionVolume() before setting
  volume.

CRs-Fixed: 360973
Change-Id: I02ac979bf83c4f3589deb5c83ae816179735aa2b