Bluetooth: MAP: Disconnect mns client when remore disables Notification

Add change to send disconnect request for the MNS session when
the remote device disables notification. currently we were only
unregistering the content observer and not sending disconnect,
which was causing PTS test case to fail.

CRs-fixed: 561658

Change-Id: Ie195b4505b4501461f7456e42245ce39fd939e79
(cherry picked from commit 599ccec713c4e337512a5147e86932946e04d88b)
(cherry picked from commit d19f31e698a3e31ae589d49a92539332407755ba)