Update screen pool connector ID ordering before adjusting desktop containments

Summary:
Setting the screenpool primary screen changes the mapping of ID ->
connector. We need this to be done before the
DesktopView::setScreenToFollow is called because otherwise it will save
lastScreen with the wrong ID.

CCBUG: 370711

Test Plan:
Hot swapped primary, both things swapped.
Restarted Plasma
Things restored as I think they should be.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3098