Gets wallpaper color from live wallpaper to be used for workspace preview

- Curretly we are using the wallpaper color of wallpaper thumbnail asset for static and live wallpaper, but the content of live wallpaper may be different from thumbnail, so it would show the wrong color for the workspace preview.
- For live wallpaper, we have to use the wallpaper color from live wallpaper instead of thumbnail. The static wallpaper would still keep using the wallpaper color from thumbnail.

Before:
https://screenshot.googleplex.com/yJCGCw2A03L.png
https://screenshot.googleplex.com/RdRQeoG0AnB.png
https://screenshot.googleplex.com/84KmcO3rTOJ.png
After:
https://screenshot.googleplex.com/5wVOM72CJTD.png
https://screenshot.googleplex.com/Psr9uLE396f.png
https://screenshot.googleplex.com/AxMsoo3F7Vc.png

Test: Manually
Fixes: 159195012
Change-Id: I439e06ff2cb19254455a698742798fd0eb8f7b39