Added internal API to get/set the A2DP Active Device

1. Call BluetoothA2dp.setActiveDevice(BluetoothDevice device) to set
   a connected A2DP device as active.
2. Listen for BluetoothA2dp.ACTION_ACTIVE_DEVICE_CHANGED intent
   that will contain the latest active device (in EXTRA_DEVICE field).
   If the active device could not be changed, the EXTRA_DEVICE
   field could be null.
3. If setActiveDevice() is NOT in-progress, BluetoothA2dp.getActiveDevice()
   can be used. If setActiveDevice() is in-progress, the result is undefined.
4. BluetoothA2dp.setActiveDevice() could be called by some other parts of
   the system, so interested parties should always listen for
   BluetoothA2dp.ACTION_ACTIVE_DEVICE_CHANGED intents and prepared
   for active device updates.

Bug: 71555243
Test: Manual
Change-Id: I860d1aba716d75856c3e9e022948a1b4b3be18bf