Fix AvrcpControllerStateMachineTest NPEs on calls to service

The way services are stopped and started for tests was refactored some
time ago. It seems we forgot to hook up our mock A2DP and AVRCP services
during that refactor.

As well, the old method started a real service. When AVRCP Controller
started, it started up our BluetoothMediaBrowserService too. Our tests
all reference this object.

This change sets mock service instances for A2DP and AVRCP, and starts
up a real BluetoothMediaBrowserService.

Bug: 332900248
Flag: EXEMPT, test change with small mechanical refactor in AVRCP
Test: atest AvrcpControllerStateMachineTest
Change-Id: I27e1012d9513c52a98d52b1b796845f542a69a97