WallpaperCropper: load wallpapers in the background

Change-Id: I134d94a85f25bf5db4c15fa9a89265c53e5b0c1d
Signed-off-by: Roman Birg <roman@cyngn.com>
HeadsUp: Fix dismiss on back key press.

Change-Id: Ia213aeba114d153f5561622a2bea884963f3d8a1