audio: Added lock during closing of direct output

- Lock is missing when direct output is closed in
  closeOutput_nonvirtual. As mDirectAudioTracks is
  being modified in this function, we need a lock
  here to avoid synchronization issues when it is
  accessed from other methods in AudioFlinger
- Fix is protect direct audio track specific block
  with mLock

CRs-fixed: 611654

Change-Id: I88f489c3bbe21ecb198bd836c05b0bfa5826d3c6