Set VPN as the preference network for specific apps

Set VPN as the preference network for specific apps when
IKEv2VpnRunner is running, and reset it when IKEv2VpnRunner is
cleaned.

VPN will be disconnected when its underlying network is gone. To
prevent packets go throgh underlying network when underlying
network is back but VPN is not connected yet, set VPN as the only
preferred network for specefic apps.

Bug: 231749077
Test: Create a test app to register default network and check if
      the VPN is the only default network for the test app.
Change-Id: Ib1b65d142a5ccc935f3882f5665134a56277f19a