Integrate testNetworkService and Manager with Connectivity stack

This change adds TestAPIs for tests to retrive an instance of
ConnectivityManager, allowing it to build test TUN interfaces, as well
as test networks.

This also integrates the TestNetwork types with ConnectivityManager,
creating virtual networks if the network agent is a test agent.

Bug: 72950854
Test: Compiles, CTS tests using this passing correctly
Change-Id: I741ef9cdf4bd4125d9129af3a030edf32f438e4f