Fix WallpaperManagerInternal NPE

Some targets don't have a WallpaperManager, thus this null check is
necessary to avoid crashes. A crash was detected in Android auto
emulator.

Fixes: 280883891
Test: build completed successfully
Change-Id: Ie475e9ba1999f109572252303e4103e5f977b4bd

services/core/java/com/android/server/wm/ActivityTaskManagerService.java

@@ -3555,7 +3555,10 @@ public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
                 mRootWindowContainer.forAllDisplays(displayContent -> {
                     mKeyguardController.keyguardGoingAway(displayContent.getDisplayId(), flags);
                 });
-                getWallpaperManagerInternal().onKeyguardGoingAway();
+                WallpaperManagerInternal wallpaperManagerInternal = getWallpaperManagerInternal();
+                if (wallpaperManagerInternal != null) {
+                    wallpaperManagerInternal.onKeyguardGoingAway();
+                }
             }
         } finally {
             Binder.restoreCallingIdentity(token);