Mark findViewById(int) @Nullable

Change-Id: I9bf7c08a896bd9c28400ff832179abc579fd502f
diff --git a/core/java/android/view/Window.java b/core/java/android/view/Window.java
index 1456b52..8964862 100644
--- a/core/java/android/view/Window.java
+++ b/core/java/android/view/Window.java
@@ -984,6 +984,7 @@
      *
      * @return The view if found or null otherwise.
      */
+    @Nullable
     public View findViewById(int id) {
         return getDecorView().findViewById(id);
     }