Stop using VPNs in getDefaultNetworkCapabilitiesForUser.

This was the last place in CS that fetched underlying networks
from Vpn. Therefore, delete Vpn#getUnderlyingNetworks, which is
no longer used.

Bug: 173331190
Test: new tests in ConnectivityServiceTest added in previous CL
Change-Id: Iec22ff636de0c02193576fe2e2d36b8bc7380457