messageTray: Do not add the tray unconditionally to ctrl-alt-tab

Commit 44851703 added the message tray unconditionally to
the Ctrl-Alt-Tab popup, but while this makes sense for a normal
session, we do not want it in the login screen.
Be a bit more careful where we make the tray available.

https://bugzilla.gnome.org/show_bug.cgi?id=685914