Correctly compute the minimum crop size

The integer division is wrong, it will round down the sample size and
make the crop larger than actually necessary.

Flag: com.android.window.flags.multi_crop
Bug: 355665230
Test: check the wallpaper sample size in logs
Test: atest WallpaperCropperTest
Change-Id: Id72a2f99e0d93c115462ea315ee83bc38c523bf8