Newly connected A2DP/HFP device will become the Active device

* Now all the logic for selecting the A2DP/HFP Active device
  is handled by the ActiveDeviceManager
* Changed the semantics of selecting the A2DP/HFP Active device:
  The newly connected A2DP/HFP device will become the Active device
* Updated the description in ActiveDeviceManager to describe the
  latest logic
* Removed old code for MESSAGE_SELECT_ACTICE_DEVICE_TIMEOUT that
  is not needed / relevant anymore.
* Updated HeadsetService, so it doesn't choose proactively
  the first connected HFP device as the Active device - now this
  is done by the ActiveDeviceManager.
  Also, updated the corresponding unit tests that were assuming
  the first connected device automatically becomes the
  Headset Active device.

Bug: 77191556
Test: Manual - connect multiple Headsets
Change-Id: I9e8c6a3493d15d710cdeb6e56c76f3dfb4abca61