a2dp: Notify about active device change when A2DP audio device is added

Motivation for such change is ommision of transition to primary speaker
device while switching from LE Audio Bluetooth device to A2DP Bluetooth
device.

Every Bluetooth device change have two interesting for this case steps.
First step is to notify AudioManager about device being available.
Second step is to notify about device activity. Previous implementation
first notifies about device activity which in fact is not available in
AudioManager, what leads to disconnection previous device (LE Audio).
In such moment there are no available Bluetooth devices in AudioManager.
Lack of Bluetooth device in AudioManager leads to primary speaker device
switch.

Checked solution assumes that A2DP device would be available for
AudioManager then from AudioManager add device callback a notifiaction
will be sent. This soultion would protect from short switch to primary
speaker device.

Tag: #feature
Bug: 284432213
Test: atest A2dpServiceTest
Change-Id: I8f7905633dc7a19654f3b6455ee2e92c550d96da