Add Settings support for Bluetooth Multi-A2DP and Multi-HFP

When there are multiple connected A2DP/HFP devices, if a connected
device's name is clicked on, that device will be chosen as
Active - i.e., it will be the device chosen for audio out / phone call.

Also:
 * Listen to the BluetoothA2dp.ACTION_ACTIVE_DEVICE_CHANGED
   and BluetoothHeadset.ACTION_ACTIVE_DEVICE_CHANGED intents
   and update the the status of the current active device.
 * When connecting a new device, and Multi-A2DP is enabled, don't
   disconnect the currently connected device.
 * Update the implementation of isA2dpPlaying() so it correctly checks
   all connected devices, not only the first one.

Test: Manual: multiple connected A2DP devices, and selecting each as
      the Active Device.
Bug: 64767509

Change-Id: I69f3c85ebf5a7f07f6deed484c6dd65705460ae4
Merged-In: I69f3c85ebf5a7f07f6deed484c6dd65705460ae4
(cherry picked from commit 1af33a19)