Unused API should be hidden.

Change-Id: Ifed4d06e620c990ccecc1ee1aaf3adedbdc7c2da

Author: dsandler

core/java/android/app/Notification.java

@@ -417,6 +417,7 @@ public class Notification implements Parcelable
     public static final String KIND_EVENT = "android.event";
 
     /**
+     * @hide
      * Notification type: promotion or advertisement.
      */
     public static final String KIND_PROMO = "android.promo";