Draft: extra-repos/systemd/systemd-services: apply upstream changes to plasma-mobile service file

I've upstreamed our service file to start the session to Plasma Mobile https://invent.kde.org/plasma/plasma-mobile/-/commit/381bfc3e0260801f71c1d421db77d717eac57f93

In the review a few changes were requested which I'm now also applying here.

Note that the systemd version we currently ship (256) has a bug breaking the Alias in the [Install] section. https://github.com/systemd/systemd/issues/33411 This is fixed at least since 256.3 so this can't be merged before we upgrade our systemd version.