save the containment only aftyer switch

Summary:
during a type switch
if the new containment gets saved before getting a view,
it will get lastScreen=-1 so the next boot
it will create a new containment

Test Plan:
switched repeatedly between desktop and folderview
the config file always got with the correct lastScreen=0 entry

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2966