Always Display Emergency Priority Banners

We have a small problem right now where we need to make sure that people
have seen a banner (regarding VE) but probably still have hide cookies.

This patch tells the CN controller to always attempt to show an emergency
priority banner. This also means that the banner itself must now track its
hide/show status to avoid bugging people if that's required.

Change-Id: Iaf0de3c7043b00a6ce02d7bff95f5d618dad8fe1
Merge "Always Display Emergency Priority Banners"