VPN: cancel always-on notification with new id

Previously we used 0 for the id of the "Always-on VPN disconnected"
notification, as did many other system services. In a recent CL, an
effort was made to assign a dedicated id to each of these system
notifications. Unfortunately, this CL only changed the id in the call to
NotificationManager#notify(), but not NotificationManager#cancel(). This
causes the notification to persist even when always-on VPN is turned off
by the user.

This CL changes the id in the cancel() call, thus fixing the issue.

Bug: 37175930
Test: manually enable and disable always-on and observe the notification
Change-Id: I39c7325c78479afe3dc9e1764d54406bd721b067

services/core/java/com/android/server/connectivity/Vpn.java

@@ -1287,7 +1287,7 @@ public class Vpn {
         try {
             final NotificationManager notificationManager = NotificationManager.from(mContext);
             if (!visible) {
-                notificationManager.cancelAsUser(TAG, 0, user);
+                notificationManager.cancelAsUser(TAG, SystemMessage.NOTE_VPN_DISCONNECTED, user);
                 return;
             }
             final Intent intent = new Intent(Settings.ACTION_VPN_SETTINGS);