audio: Fixed dead lock in AudioService during volume change

- AudioService goes into dead lock when there is a volume
  change for source stream and the respective aliased stream
  simultaneously from multiple threads
- When setIndex is called for source stream, it also updates
  the index for the aliased streams. Meantime, setAllIndexes
  is called on aliased stream. Initially, it gets index of
  default device of source stream. It requires lock on the
  source stream. This is leading to a dead lock
- Fix is to avoid locking parent while updating child
  streams. Removing lock from method and synchronizing
  parent only when it is updating member variable.

Change-Id: Id38e539e2ec178ea25548312a1dd62d2de3043da
CRs-fixed: 635789