Change default wallpaper plugin to org.kde.color

Summary:
This is to implement the new as described in T3364. The background
should be just a plain color, thus using color wallpaper plugin by
default.

Test Plan: Locked screen, wallpaper is color

Reviewers: Plasma, VDG

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2352