[kded] Cleanup KScreenDaemon class

Summary:
Do not export class, privatize and unvirtualize members. Rename
config init function, cleanup includes and white space.

Test Plan: Manually in Wayland session.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: broulik, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16875