Add shell TestNetworkManager and Service

This adds the scaffolding for TestNetworkManager and TestNetworkService.
These are separate from the ConnectivityManager/ConnectivityService in
order to prevent polluting the API space, and making them harder to
maintain.

Bug: 72950854
Test: Compiles, CTS tests relying on this pass
Change-Id: Ic42c09a33d962471e2587ec27f3fab2d72bbcd42