Add a workaround to play A2DP SBC Mono

There is a similar WAR of aosp/522661 at A2DP legacy HAL. In order to
suport MONO channel mode, the PCM audio is pulled as STEREO and mixed
into MONO by the Bluetooth Audio HAL.

Test: Playing SBC mono with Headset
Bug: 127593318
Change-Id: I78f3973ba6c8c733dc18122288a915daed97be65