Fix build.

Per comments in http://ag/704632 , purgeResources was intended to
be hidden.

Bug: 18386420
Change-Id: Iced006878f86c6b7b15dd38f5e704a2f97aeeda5

core/java/android/app/Notification.java

@@ -3797,6 +3797,9 @@ public class Notification implements Parcelable
             return wip;
         }
 
+        /**
+         * @hide
+         */
         public void purgeResources() {}
 
         // The following methods are split out so we can re-create notification partially.