Set ExitType when running applications as transient systemd services

KIO internally has 3 paths to launch applications.
 - fork and exec
- fork and exec then put in a "scope" so that we have our own cgroup
(default)
 - asking systemd to launch the application

The latter was explicitly disabled by default because we had some issues
with applications being cleaned up when the first process exited. We
made those upstream changes a year ago, but haven't adopted the changes
here. This brings it in line with the setup used by xdg-autostart-generator.

Nothing will change when we land this as this operation mode is not
enabled.

CCBUG: 461289
CCBUG: 423756


(cherry picked from commit 5c0ce0f5)