Adapt to new KScreenLocker API

Summary:
KScreenLocker only anymore takes an FD when it tries to lock. The connection
is created and kept internally in KWin.

At the moment we do not do any further checks on the lock but directly hand
over an FD whenever KScreenLocker is about to lock.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, apol, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D28085