Don't setup PlasmaWindowManagement integration for lock screen windows

Summary:
We don't want external processes to know anything about the lock screen
windows. Especially we don't want them to be able to request close on
them.

Thus better never show thus windows to them.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D1760