We currently special-case the DISABLED error when initializing
filtering, but not on app filter changes.
While it seems reasonable that Malcontent.Manager wouldn't emit
the signal while disabled, that's not actually true: It is emitted
when any user account information tracked by AccountsServices
changes.
Even if the signal were limited to changes of the ParentalControls
extension, it would still get emitted when app filtering *becomes*
disabled.
So regardless of potential improvements in libmalcontent itself,
we should filter out the DISABLED consistently, both when creating
the initial filter and when updating it.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6749
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2796>
593f659a73