[AudioManager] Fix AudioStreamPolicy and AudioVolume errors

[AudioManager] Fix AudioStreamPolicy and AudioVolume errors

- Setting current volume type to none should unset the current type.
- Completed event implementation in AudioStreamPolicy constructor.
- sound_manager_set_focus_state_watch_cb() can only be called once.
  So use "all" mask and filter callback events and invoke.
- Removed unnecessary 'using' statements.

Signed-off-by: Sidharth Gupta <sid92.gupta@samsung.com>
Change-Id: I948760d66a08c3c54d18deef1d1571c415dc9c35