Show notification for always-on app VPN

This is the same notification as the one shown during legacy lockdown
mode, sans the 'reset' button.

The notification is only shown during times when VPN has not yet
established or has failed, for example during boot or after a crash.

Bug: 29123115
Change-Id: If0e2eb4f8fb220a21d6d462363a05869e27c7b6e