Media session should start on audio focus

Problem: Currently, media session is activated immediately once a
device is connected and gets deactivated when a device is disconnected.
However, this results in clients of media session to create controllers
for the respective active media session even though BT audio focus was
not requested. An active controller results in media events to be sent
to bluetooth.

Solution: Bluetooth requests audio focus when the media service is
prepared. Only once the audio focus is granted, bluetooth module will
activate the media session. Similarly, media session will be deactivated
when the audio focus is lost. On phone being disconnected, media session
will also be de-activated.

Bug: 262309425
Test: Unit-test to validate media session is deactivated on different scenarios.
Tested on device to validate media & assistant use-cases.

Tag: #stability
Change-Id: I90242c92d537bf0e7bc4c30749811af09e5821e5