Let wallpaper know when to animate AoD transition

Sometimes the screen will blank, and sometime the
wallpaper has the opportunity to animate the
transition.

Bug: 64155983
Test: atest tests/Internal/src/android/service/wallpaper/WallpaperServiceTest.java
Test: atest packages/SystemUI/tests/src/com/android/systemui/doze/DozeWallpaperStateTest.java
Change-Id: Ia92c00edb98eeeba42da33bdc7bec3feb961a658