Support wallpapers in AoD

Such wallpaper has to define supportsAmbientMode,
and set it to true on its android.service.wallpaper
meta data.

Also introduces WallpaperService.Engine#onAmbientModeChanged
to notify a live wallpaper that the display state has changed.

Change-Id: I49e846069a698b3cc3bb6e7cda98172920eaae4c
Bug: 64155983
Test: runtest -x frameworks/base/packages/SystemUI/tests/src/com/android/systemui/doze/DozeWallpaperStateTest.java
Test: runtest -x frameworks/base/tests/Internal/src/android/app/WallpaperInfoTest.java
Test: runtest -x frameworks/base/tests/Internal/src/android/service/wallpaper/WallpaperServiceTest.java
Test: set AoD wallpaper, go to aod, lock screen, launcher
Test: set regular wallpaper, go to aod, lock screen, launcher