HFP AG: Broadcast disconnected state while BT is turning off

When BT is turning off, broadcast the disconnected state
for connected devices so that other listeners whoever interseted
in knowing the BT state are aware of BT has been turn off.

CRs-Fixed: 1058006
Change-Id: I2b25ec059cf2ea7de23d597c6ba6e75610cdc3d5