MAP: fix problem with MNS not disconnecting correctly

MNS is needed to disconnect with the MSE but did not.
This patch fixes that by re-adding a disconnect() that was wrongfully removed during review.
Patch set 2:
-----------
 - Split disconnect and shutdown for MNS connection
bug: 11161674
Change-Id: Idd09fc774584dfdb171b0d00be0f7711ae57b853