Create a new API - setVpnNetworkPreference() for the caller to
set VPN as the preference network.
VPN will be disconnected when its underlying network is gone.
To prevent packets going through an underlying network when the
underlying network is back but VPN is not connected yet, set VPN
as the only preferred network for specific apps.
Bug: 231749077
Test: 1. atest FrameworksNetTests
2. Create a test app to register default network and check if
the VPN is the only default network for the test app.
Change-Id: Iabcd38e2fec2aefedbf78d20e338f222d83a9e7f
3ba7cc25c0