LiveWallpaperDownloader (3/3)

Created LiveWallpaperDownloader for downloading live wallpapers.

Test: Manually tested that downloading wallpapers works, see bug
Bug: 315572439
Flag: ACONFIG com.android.wallpaper.multi_crop_preview_ui_flag DEVELOPMENT
Change-Id: Ic2a8077814b41634464de30981c558a4abc89a8e