Unify behavior of various cases of "no underlying networks"

Before this change, VPNs having no underlying networks would be
marked as metered as the safe option, but VPNs having only
disconnected underlying networks would be marked as unmetered.
Fix this discrepancy.

Bug: 79748782
Test: runtest frameworks-net
Change-Id: I51c3badde29f43f692f383553bd98327d2da8dd1