Restructure Test Network requests for VCN connections.

This CL restructures the NetworkRequests used for TestNetworks
in UnderlyingNetworkTracker to clearly show that they only require
TRANSPORT_TEST and the relevant subIds.

Bug: 182291467
Test: atest FrameworksVcnTests CtsVcnTestCases
Change-Id: I4b135749fdde9f91c66b87b1ded921700e6440fd