Use only width to determine largest display when centering the wallpaper

Instead of comparing the largest side of one display with the largest
of the other (which doesn't always do what we expected), compare just
the width, as the offset is only applied to the x axis anyway.

Bug: 321041432
Flag: N/A
Test: atest WallpaperControllerTests
Change-Id: I2495b28d5f620b9de0d804f05dc331fa45e585fa