Add color scheme constructor that takes WallpaperColors & Style objects

Pairs with, and needed for, a CL that enables Widgets to use the user-selected theme variant.

Bug: 213314628
Test: visual inspection at runtime: use different wallpapers and
widgets in different locations, change to different theme variants,
observe changes and verify they're in line with expectations (ex.
if a widget is blue in the default theme variant, in the expressive
theme variant it rotates hue just like a blue theme color does in
expressive hue)

Change-Id: I288a4faeba06042037b468c8345414ef5547325c