Notify VPN status from SystemUI as notifications

A `VPN is connected.` ongoing notification is sent when connected to a
VPN and a `VPN is disconnected.` short lived notification is send when
disconnected from a VPN.

Test: Manual tests. Connecting and disconnecting to VPNs (from a VPN
client app) show the (undismissable, ongoing) connected and
(dismissable, short lived) disconnected notifications. A CTS test will
be added in a separate CL.
Bug: 157500504

Change-Id: I321dec45127104b5139b9e868bac76b9d882e9f2