Change ConnectivityServiceTest to prepare for VPN code move.

No-op changes to make it easier to review the VPN code move.
1. Set underlying networks via MockVpn, not ConnectivityService.
2. Add a processBroadcastForVpn utility method that we can modify
   without too much churn when the code is moved.

Bug: 173331190
Test: test-only change
Change-Id: I4256ca5f33034ef9d1de79857795a6ed1d644e61