Show a disclaimer about enabling vpn lockdown

Lockdown is now the default option, not best-effort mode. It's easier
to shoot oneself in the foot now so we'll show a warning to explain that
before switching it on.

Bug: 29052115
Bug: 29076208
Test: com.android.settings.vpn2.AppSettingsTest
Change-Id: Ia6845e6a7d57baa5476b8a021fb1255fd74aabea