Fix wallpaper not loading (leaving the background black)

We need to apply the wallpaper anchors after the lockscreen theme is loaded, otherwise they won't be set properly and we end up with a black background.

This issue was triggered by !84

The reason why this wasn't noticed on Plasma Desktop, was that its lockscreen theme draws a FastBlur above the wallpaper in the background which will still correctly display the wallpaper item.

I had forgotten about that I used that as a workaround in Plasma Mobile for months... oops