Fix theme cannot change due to there is no wallpaper colors.

Device theme cannot changed because there is no wallpaper colors when
first boot with default image wallpaper. We can still calculate it
if there is no cropFile with the wallpaper component is ImageWallpaper.

Fix: 122660786
Test: manual test.
Test: atest WallpaperColorsTest WallpaperManagerTest WallpaperServiceTest

Change-Id: I26cc1d751dadd28185c650eff6e6ef60d5986aaa