MAP: Handle possible NPE while disconnect map.

Precondition:
Verified with IOT: MecApp in MCE role

UseCase:
1. Connect MAS
2. Register Mas Instance via MNS
3. Disconnect MAS

Failure:
Bluetooth App crash sometimes when disconnect map profile.

Root Cause:
The issue is hit when MAS disconnect is finished before MNS unregister.
In this case NullpointerException is happening because of clearing
BluetoothMapContentObserver already as a part of cleanup.

Fix:
Handle Null pointer exception while last
MAS instance disconnect is handled before MNS
unregistration.

Change-Id: I4e72038a5ddc6c3ac8c515def57cb4f98fe87675