extensionUtils: Always use manager to find current extension

extensionUtils: Always use manager to find current extension

Now that we always have an extension manager object, we can use
the same code path for use from extensions and prefs.

For that, inject the D-Bus service's extensionManager instead
of the current extension.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2832>