audio_a2dp_hw: add device lock

Prevent conflict between closing a stream (adev_close_output_stream)
and setting parameters (adev_set_parameters).

Test: Manual - Bluetooth A2DP streaming and changing codec parameters
Bug: 36723276
Change-Id: Id8b9fcdf594e3d5fde139719d324420468c4c0aa
(cherry picked from commit d60edbb1)