Offset the wallpaper to center in smaller display

If we have more than one physical display, offset the wallpaper
in the smaller displays so that the center corresponds to the
center of the image in the largest display.
This behavior is controlled by a config flag to avoid
affecting the behavior of existing foldable devices.

Bug: 195120817
Test: manual (visually verified with different images)
Test: atest WmTests:WallpaperControllerTests
Change-Id: I57bfeb5ab82f265c991f3be02313abb4289d42dd