Prevent disconnecting admin-configured VPN

First, if the VPN is configured by an admin, the preference is
disabled and tapping on it will results in a policy disclouser
dialog.
In addition restriction checks in the dialog also check if the
VPN is admin-configured.

Bug: 179975048
Test: Manual, setting VPN in profile and primary user and via DPM API.
Test: make RunSettingsRoboTests -j ROBOTEST_FILTER=com.android.settings.vpn2
Change-Id: Id59d2ac2782e83601bc3093d3a092faea36ff5d9