WindowState: Correct wallpaper scaling

A recent CL moved wallpaper scaling to the WindowState level
to avoid fighting with the BLAST adapter at the WindowState level.
It mistaken scaled from the top left corner of the wallpaper instead
of the center. This CL restores the old logic.

Bug: 190532253
Test: Existing tests pass
Change-Id: Ic809e69d1a2b57b771e0657ba98b508d062e58c9