notifications: fix invalid format string

GLib-GIO-CRITICAL **: g_settings_get: the format string may not contain
'&' (key 'application-children' from schema
'org.gnome.desktop.notifications'). This call will probably stop working
with a future version of glib.

This is fallout from https://bugzilla.gnome.org/show_bug.cgi?id=719979

https://bugzilla.gnome.org/show_bug.cgi?id=721430