hal: avoid stop_output_stream() in out_stop()

- For mmap usecase, call stop_output_stream() in out_standby()
  instead of out_stop() and modify out_start() accordingly
  to handle different scenarios.
- This is required to avoid closing snd device in out_stop() and
  instead close it during standby.
- Fix KW issue. Unlock locks in route_output_stream
  while returing.

Change-Id: I3c2d396590d458e566a676c69698763608a00d6f