Define VcnStatusCallback register/unregister.

This CL defines VcnStatusCallbacks, which are callbacks used to register
for status updates to a specific subscription group. These Callbacks may
be registered with VcnManager at any time, but will only be invoked for
the specifies subscription group and only if the registering app has
carrier privileges for that subscription.

Bug: 163433613
Test: atest FrameworksVcnTests
Change-Id: Iefd284ae2d09676d195e2a12bf660be3596da59b