gnome-shell hard code freeze break request

[Colin Walters <walters verbum org>]

https://bugzilla.gnome.org/show_bug.cgi?id=691987

I'd characterize the risk of this change as "low/medium".  I tested the
change of course (in gnome-ostree FWIW) but it's a nontrivial change to
a nontrivial part of the code.

The benefit is that we're less likely to leave the user's session in an
unlocked state.