This CL integrates WallpaperModel into LiveWallpaperClient such that LiveWallpaperClient may provide a list of WallpaperModel for a give WallpaperInfo Bug: 439638172 Flag: com.android.wallpaper.refactor_individual_picker_flag Test: manual Change-Id: Ic4f4b62c10635d57370819c8d3f6888a9cb53a94