Rename WallpaperLocalColorExtractor

Now, since the new flag, this also extracts global colors, not only
local colors. Also, there are other wallpaper color extractor classes
within android, so append "Image" to make it clear it's for
ImageWallpaper.

Flag: EXEMPT IDE refactors only
Bug: 420463483
Test: presubmit
Test: atest ImageWallpaperColorExtractorTest
Change-Id: Ief9f135b79253e5a646c7ed90606dfbb4fffa0bf