Use a systemd --user service instead of xdg autostart for tails-greeter-session-helper

I’ve ported all our stuff last year, and would like to avoid new stuff added in the place that I’ve ported stuff from. See 9fb0136 for initial pointers towards how this can be done.

Parent Task: #8230 (closed)

Original created by @intrigeri on 11700 (Redmine)