Summary:
As discussed and agreed to by VDG (T7914), change the lock screen background from the blue color to the current Plasma wallpaper.

CCBUG: 381407

Closes T7914

Test Plan:
- `rm ~/.config/kscreenlockerrc`
- Build and install kscreenlocker with this patch
- Lock the screen

{F5753279}

Reviewers: #plasma, #vdg, graesslin, abetts, davidedmundson, mart

Reviewed By: #plasma, #vdg, abetts, davidedmundson, mart

Subscribers: fvogt, Fuchs, broulik, davidedmundson, zzag, Pitel, progwolff, abetts, hein, mart, graesslin, plasma-devel

Tags: #plasma

Maniphest Tasks: T7914

Differential Revision: https://phabricator.kde.org/D11308