Bluetooth: pause music player on A2dp service disconnect

When A2dp service disconnects as part of SSR, it marks a2dp state
as unavailable in Audio module but does not pause music player, as
a result when bluedroid stack restarts as part of sub system restart,
media player does not pause and music continues to play on device
speaker.
This change helps to pause the media player, in addition to marking
a2dp as unavailable, so that media player pauses when a2dp service
forcibly disconnects as part of sub system restart with A2dp in
connected state.

CRs-Fixed: 590462
Change-Id: I05ee5bcc808eb438156b64181d54e8ccabe1e186