Fix wallpaper configuration for plasma mobile

Summary:
f6ce91499a16cd9e8 replace the height/width property of Image with
targetSize, and phone image configuraiton was not adopted for that
change resulting in broken configuration window for plasma mobile

Test Plan: tested on phone

Reviewers: broulik, Plasma

Reviewed By: broulik, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4528