VPN is considered fully routed if both IPv4 and IPv6 have
either a default route or a prohibit route.
Bug: 145332510
Test: atest FrameworksNetTests
Merged-In: I59cf48552bca98092d1212e3d718fd420add5458
Change-Id: I59cf48552bca98092d1212e3d718fd420add5458