Adjust mini bitmap size for color extraction

Make sure that ImageWallpaper's color extractor rescales the bitmap
exactly like WallapperColors#fromBitmap does.

This should avoid slightly changing the colors and failing some
screenshot tests when totally moving the color extraction logic to
ImageWallpaper.

Flag: NA, currently the mini bitmap is unused
Bug: 328791519
Test: atest WallpaperLocalColorExtractorTest
Change-Id: I9f90b452106c0f6a84ee50ab0559ef868b79941b