Notify status callbacks when a VCN enters Safemode.

This CL updates VcnManagementService to notify VcnStatusCallbacks when
the VCN for their specified subscription group enters Safemode. In order
to be notified, the registering app must also have permissions for the
specified permission.

Bug: 163433613
Test: atest FrameworksVcnTests
Change-Id: I3242ad0ee1dc406aef56253f884c2544a994869e