HFP: Add APIs for set and get active device (2/3)

1. Call BluetoothHeadset.setActiveDevice(BluetoothDevice device) to set
   a connected HFP/HSP device as active.
2. Listen for BluetoothHeadset.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. BluetoothHeadset.setActiveDevice() could be called by some other parts of
   the system, so interested parties should always listen for
   BluetoothHeadset.ACTION_ACTIVE_DEVICE_CHANGED intents and prepared
   for active device updates.

Bug: 68951996
Test: runtest -j40 bluetooth
Change-Id: I4471a35aa17f369bef49fbdb6f3ccaee393013b9