MAP: Fix MNS observer sync and sendevent exception issue.

Allow MNS Client sendEvent only when MNS Client is registered
and connected to fix NullPointer exceptions. Mark MNS disconnect
and observer registration as synchronized to handle synchronization
for operations from worker and main threads.

CRs-fixed: 571864
Change-Id: If34a6b67a1774f693ed42cfb6c871cd9563abe55
Bluetooth: Check A2dp device priority.

Add support to update stack for A2dp priority
of device.

Change-Id: I65971ffc68e055d68091f73a489a8d8c69c9420b
CRs-Fixed: 510957