Fix volume control failed when switching active a2dp device

Add AudioManager.FLAG_BLUETOOTH_ABS_VOLUME when mute/un-mute stream
volume to avoid AudioService update volume to AVRCP during switching
active device to avoid wrong volume level setting.

Bug: 132416679
Test: Manual swicth a2dp device.
Merged-In: Idc7a701c754edfe87ba72bb83d6b3809572eaaa5
Change-Id: Idc7a701c754edfe87ba72bb83d6b3809572eaaa5