WallpaperColors refactor

Hiding color extraction into WallpaperColors.
This enables us to create WallpaperColors from a a Bitmap
or Drawable.

Fixes: 62197187
Fixes: 62490115
Test: runtest --path cts/tests/app/src/android/app/cts/WallpaperColorsTest.java
Change-Id: I614cfa205e02b551a141642eac6de21251c3bff6