Additional Software: Make tails-additional-software-config D-Bus activatable

This makes the app accessible via Dogtail.

According to GNOME documentation [1], this is the recommended way to ensure
that an app is a direct descendent of the session, including that it
ends up in the correct cgroup.

[1]: https://wiki.gnome.org/HowDoI/DBusApplicationLaunching#Why.3F

Closes #19233