Some battery improvements:

- New API for iterating over history that will allow a better implementation
  in the future.
- Now do writes asynchronously.

Also improve the documentation for Activity.onRetainNonInstanceState().

Change-Id: Idf67f2796a8868eb62f288bcbb2bad29876c8554

services/java/com/android/server/WindowManagerService.java

@@ -8316,6 +8316,11 @@ public class WindowManagerService extends IWindowManager.Stub
             return;
         }
         
+        if (mDisplay == null) {
+            // Not yet initialized, nothing to do.
+            return;
+        }
+
         boolean recoveringMemory = false;
         if (mForceRemoves != null) {
             recoveringMemory = true;