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