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

* Add set active device APIs to set active device from Java layer
* Set active device to the first connected device if no one set it
  before
* Clear active device on RFCOMM disconnection if active device is the
  one getting disconnected
* Currently, the active device value is not used in any functional code

Bug: 68951996
Test: runtest -j40 bluetooth
Change-Id: Id0c00e89178b0f963ed8642e72c7010160dd1d68