A2DP Offload support

The encoding of the A2DP data packets if offloaded
to platform specific HAL. The communication with the
HAL is enabled by IBluetoothAudioOffload and
IBluetoothAudioHost interfaces. Bluetooth stack is
still responsible for A2DP control path.

Bug: 72242910
Test: Manual; TestTracker/148125
Change-Id: Ie6e9c233ca431d0ab8cf15d10c30836e41bbdcb7