Have DisplayContent be the call point for adjusting wallpaper windows

4th step in trying to make the WindowList private to DisplayContent
Have the rest of the system call DisplayContent when they need to
adjust the position of the wallpaper windows in the window list
instead of calling WallpaperController directly. That way the display
content can control the window list that the wallpaper controller sees.

Test: Existing tests pass and manual testing.
Change-Id: Iaa7f421d7cd24d36e5a83e091f77b4a08d0ae123