Use the default Plasma wallpaper on the lock screen

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

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