Using the user object at `/org/freedesktop/login1/User/self` is
convenient, but has the caveat that login1 does not emit the
`PropertiesChanged` signal for the object.
That is indeed logical, as for signal emissions there is no
sender that can be used to resolve `self`.
The new TimeLimitsManager depends on change notifications for
user properties, so stop using the `self` shorthand and instead
create the User proxy for the user's UID.
Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8185
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3636>
5115236703