Wallpaper Improvements CLs (2 of 3): New Overlay Wallpaper

Updated default overlay wallpaper file to be PNG file with 1 pixel of
Hex RGB: #212121.
"#212121" is the value currently used by LandscapeWallpaperService,
which is being removed in favor of this static image solution.

b/37993879

Test: Manually

Change-Id: I11d782aa6f3e294b84293c0a5191e28e24b2904c