Update docs for Window.setElevation(), apply changes immediately

BUG: 18983689
Change-Id: I471104092d6296dc0ba2689da96783be088fbce8

core/java/android/view/Window.java

@@ -1087,6 +1087,10 @@ public abstract class Window {
 
     /**
      * Sets the window elevation.
+     * <p>
+     * Changes to this property take effect immediately and will cause the
+     * window surface to be recreated. This is an expensive operation and as a
+     * result, this property should not be animated.
      *
      * @param elevation The window elevation.
      * @see View#setElevation(float)

policy/src/com/android/internal/policy/impl/PhoneWindow.java

@@ -1338,6 +1338,7 @@ public class PhoneWindow extends Window implements MenuBuilder.Callback {
         if (mDecor != null) {
             mDecor.setElevation(elevation);
         }
+        dispatchWindowAttributesChanged(getAttributes());
     }
 
     @Override