Fix small computation error.

The case "BALANCE" is usually used to find a rotated crop, i.e. in cases
where the reference crop is for display "x, y" and we're looking for
"y, x". The previous computation for widthToAdd worked only in this
specific case (when screenRatio = crop.height/crop.width), but the
formulae was globally wrong. Fix that and add a note.

Flag: aconfig com.android.window.flags.multi_crop TEAMFOOD
Bug: 281648899
Test: atest WallpaperCropperTest
Change-Id: I70d1b1a18645ac0a161645d7a2d016a47b61c5c8