notificationDaemon: Fix grouping by PID

notificationDaemon: Fix grouping by PID

For fd.o notifications, we are taking the sender's PID into
account when associating notifications with sources (mainly
to deal with notify-send).

This broke when the implementation under the well-known name
was moved into a separate service, as the implementation in
gnome-shell will now always see the public notification-daemon
as sender.

Restore the old behavior by resolving the sender PID in the
separate service, and pass it as hint to the implementation
in gnome-shell.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2592