Bluetooth: Validate SCO device.

This change makes sure that active
SCO device is vaidated before using it
to send the volume changed event to stack.
A NPE happened when audio updated the SCO
volume changed event at the time when
SCO was disconnected and the SCO
active device was null.

Change-Id: I56a2b05a832ae01dad9f325baad79fd46640c81c
CRs-Fixed: 752481