Call VcnStatusCallback#onVcnStatusChanged on register.

This CL updates VcnMangementService to notify VcnStatusCallbacks on
registration with the current status of the VCN for the specified
subscription group.

Bug: 180659281
Test: atest FrameworksVcnTests
Change-Id: Id2c74e855fa12d21d292ee94a72ad047f2d56aca