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