Bluetooth: MAP: Disconnect mns client when remote disables Notification

Add change to send disconnect request for the MNS session when neither
email nor sms/msm observers are registered and remote device disables
notification. currently we were only unregistering the content observer
and not sending disconnect, which was causing PTS test case to fail.

Change-Id: Ib87d6b92cc0390c6fc1d7bda1c0e70197ae3606d
CRs-fixed: 594864