Standby A2DP audio hardware interface when disabling A2DP.

Patch supplied on advice of partner. This causes us to send suspend_sink to
Bluez via socket interface, so we enter suspend on the A2DP link faster.
This is especially important when switching to SCO so that we come closer to
whitepaper recommendations to suspend A2DP before setting up SCO.

We have another patch set to add DBUS A2DP suspend and resume calls to Bluez
that will do a better job of following whitepaper recommendations for
A2DP -> SCO -> A2DP, but this small patch is still an improvement.