Expose APIs for VcnStatusCallback.

This CL exposes the APIs needed to define VcnStatusCallback as well as
register and unregister them. VcnStatusCallbacks should be registered by
Carrier Apps to receive VCN-related status updates for their configured
VCNs. Potential status updates include a VCN entering safe mode or an
error with an underlying Network.

Bug: 163433613
Test: atest FrameworksVcnTests
Change-Id: I1bf01139d4feea71f650acdbf98c6634c66acbbf