Add navbar background preview to ThemesContract

Change-Id: Id3ed43c86649b1757fec7f3fc7dee2d2730258e2
WallpaperCropper: fix lockscreen wallpaper "Set wallpaper" string

The string is wrong and does not reflect the correct action the user is
about to apply.

Change-Id: I91eba87a38546563d6cf88138a0ed07219ec794f
Signed-off-by: Roman Birg <roman@cyngn.com>