Fix issue 2604270 Device auto connects to Car Dock (Media profile) after undocking.

There was a regression introduced by change 4c637b9e
fixing issue 2578813. As the A2DP device disconnection is notified to the audio policy manager
after a delay, there is a period during which the A2DP device is actually not connected but the
A2DP output is considered present by audio framework. If a playback is started during this period,
The A2DP output requests the activation of the A2DP sink which in turn triggers a reconnection
of the A2DP sink.

The fix consists in suspending the A2DP output immediately when receiving the A2DP disconnection
intent so that any activity on A2DP output is ignored until the output is actually closed by the
audio policy manager.

Change-Id: I16eaace45e12bfc970f50836b46a73b756b493f0