Add explicit init/cleanup steps for A2DP Source and Sink

The startup/shutdown steps for A2DP Source and Sink
are done when setting/resetting the Active device, and this
implies the device has been connected.

The operations that must be done before the remote device
is connected are moved to init (and cleanup for the reverse operations)

Bug: 73832547
Test: Manual - Connect/disconnect/play A2DP to Headset
Change-Id: Iaa55c2d27443646111f07a53d2c5163bca9e256d