Properly cancel Tether notification.

When we add a second type of tethering we unify the notifications
into a single generic type.  This is done by canceling the first
and replacing it, but this flow was improperly canceling, so
you could end up with orphaned tethering icons.

bug:7283605
Change-Id: I1d136f51592b4326d48578cf67b69122e45d4984

Author: Robert Greenwalt

services/java/com/android/server/connectivity/Tethering.java

@@ -454,7 +454,8 @@ private void showTetheredNotification(int icon) {
             if (mTetheredNotification.icon == icon) {
                 return;
             }
-            notificationManager.cancel(mTetheredNotification.icon);
+            notificationManager.cancelAsUser(null, mTetheredNotification.icon,
+                    UserHandle.ALL);
         }
 
         Intent intent = new Intent();