meta-launcher: Explicitly look up object path for seat

If mutter is running as a systemd user service, then we cannot use the magic "self" session for the ID lookup. For now we need to lookup the ID explicitly. Eventually we can change to use the magic "auto" paths for both the session and seat, but that will require systemd v243.

Relevant systemd MR/comment: https://github.com/systemd/systemd/pull/12424#issuecomment-487962314

See also gnome-session!13 (merged)