Wallpaper color extraction

Now it's possible to listen to changes on wallpaper colors by
registering a listener on WallpaperManager. It's also possible
to know the current wallpaper colors and if it's light or dark.

Test: runtest --path cts/tests/app/src/android/app/cts/WallpaperColorsTest.java && \
      runtest --path cts/tests/app/src/android/app/cts/WallpaperManagerTest.java
Bug: 36856508
Change-Id: Ia6b317b710e721d26f0fe41c847b9426e61d8d8b