Let the background color of the previews can be adapted by different wallpapers.

Remove the hardcoded background colors and let the background colors respect the theme's android:colorBackground attribute.

PS: Need to follow the steps in go/usingmonet to let the background color can change with the wallpapers.

Screenshot:
https://screenshot.googleplex.com/3K5W5BGG2tFBxnb.png

Bug: 173549815
Change-Id: I090fa9eb2303c78959f44649cfb28d1346dcf551