shell: Ensure containments that belong to the current activity are not deleted prior to loading

I am not sure if the behaviour for KActivities is intended here, but no activities are reported for this check for some reason (so this may not be the best fix).

We have had an issue for a few months now with Plasma Mobile where the wallpaper and homescreen would always get reset on every boot, this seems to fix it.

See plasma-mobile#222