Add testLockdownVpn that mocks platform VPN.

Add a test for lockdown vpn that uses TYPE_IKEV2_IPSEC_PSK and
mocks platform VPN by override in startLegacyVpnPrivileged().
In the context of ConnectivityService, setVpnDefaultForUids()
is the main interaction.

Refactor testLegacyLockdownVpn to take a VpnProfile and assert
behaviors with and without setVpnDefaultForUids().
This includes:
    1. Updating callback asserts and assertActiveNetworkInfo to
       reflect setVpnDefaultForUids().
    2. Adding TODOs where mCm.getActiveNetworkInfo() returns
       unexpected values.

Bug: 230548427
Test: atest FrameworksNetTests
Change-Id: Ida4a4bc745af5ba2fc251795b2ffca56ead79b7f