Move wallpaper crop logic in a new class WallpaperCropper

Part of the WallpaperManagerService refactor. This mainly moves the
method generateCrop and generateCropInternal

Test: treehugger
Test: atest CtsWallpaperTestCases
Test: atest FrameworksMockingServicesTests:WallpaperManagerServiceTests
Bug: 264637309
Change-Id: I0446e677de0c673cf5cfbd704a1dac7d529906ff