extra-repos/systemd/systemd-services: add a service file for waydroid-session

A thing to figure out: in Alpine we ship a file in /etc/xdg/autostart to autostart the Waydroid session. This service file is supposed to replace that functionality but now with that present the session will be started twice (or at least it tries too). We somehow need to get rid of that file without breaking the functionality on Alpine without systemd.

EDIT: never mind, of course we can just do this the same like we do for other applications like PipeWire: x-systemd-skip=true in the desktop file. I just made that change in Alpine, this is good to go now.