messageTray: Ignore showBanners policy for critical notifications

messageTray: Ignore showBanners policy for critical notifications

The critical hint is meant to be used for notifications that must not
be missed - running out of battery being the prime example - so it
makes sense to ignore the policy in that case and make sure to always
show them to the user. This is consistent with blocking normal
notifications while showing a fullscreen window, but letting critical
ones through.

https://bugzilla.gnome.org/show_bug.cgi?id=779974