gnome3.gnome-characters: correct entry point

By wraping the files, the `*-.wrapped` file will be recognized as the
entry point, which is used as a base for loading additional files so
we need to override the entry point function.

We also ensure BackgroundService gets wrapped with wrapGAppsHook.

Fixes #31168

Co-authored-by: Jan Tojnar <jtojnar@gmail.com>