launch-environment: Use new gnome-session AutostartWhitelist
See: gnome-session!41
We really just want a whitelist of applications to be started through autostart. Use the new AutostartWhitelist key file to list application IDs that may be autostarted (while all others are not considered for autostart).
The main disadvantage is that all applications that potentially should be autostarted need to be listed here (as part of distribution patches).
In the common case, spice-vdagent should be added to the list.