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.

Clean cherry-pick of ag/4113809

Bug: 79748782
Test: runtest frameworks-net
Change-Id: Ie6ace6bd95139605ffcfa8cd6c15cf28f8fa28c8
Merged-In: If19b85325e7d684e645470293b3c8a674084c641
Merged-in: I22f80a6a39d4a19ff74aa61fcbd66f1a041b1003