shell: Fix crash in polkit agent

The polkit agent does not take ownership of the request's cancellable,
so it may no longer be valid after the request finished and we need
to disconnect it before that. Before the GTask port in 8b4249ef,
it was safe to do that when freeing the request data because requests
completed in an idle, but now we need to do it before returning the
task result.

https://bugzilla.gnome.org/show_bug.cgi?id=757668