accounts-service: Allow installation to the system repo by default

Note that this permissions check only concerns parental controls. If the system flatpak polkit policy disallows installation to the system repo (or requires an administrator password for it), this setting cannot override that.

The default was previously to disallow installation, because the flatpak polkit policy was previously overridden by these settings. How parental controls are checked in flatpak has evolved since then, though.

See: https://github.com/flatpak/flatpak/issues/3995

Signed-off-by: Philip Withnall pwithnall@endlessos.org