Make the virtual keyboard disappear when locking the screen

Summary: Previously, the virtual keyboard did not close when locking the screen, instead blocking part of the lockscreen while not actually working. This patch actually makes the keyboard close before locking

Test Plan: Open virtual keyboard -> Lock Screen -> No keyboard shown

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, apol, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27172