Allow tests to create TRANSPORT_TEST|TRANSPORT_VPN networks.

This CL allows an app that has the MANAGE_TEST_NETWORKS
permission to create test VPN networks.

The code enforces that such networks can never apply to any UIDs
and thus will never carry any traffic.

Bug: 173331190
Test: passes existing tests, moved tests pass
Change-Id: I5befea0e3b4b6dce4ca0c6a04471a055186b644c