Switch to using status codes for Vcn

This change adds status codes, and switches the Vcn class to use these
to signal safe mode status. Additionally, this allows a distinction
between a VCN that is quitting and one that is simply in safe mode,
providing potential for an opportunistic safe mode where the VCN will
continue to attempt to serve NetworkRequests while not restricting
underlying networks.

Bug: 183174340
Bug: 181789060
Test: atest FrameworksVcnTests
Change-Id: I9f2b2d0d8d7b3ade19ca5adcd48cc920171bf8c3