Clear VPN config when VPN disconnects.

This ensures ConnectivityService.getVpnConfig() returns null after a
VPN disconnects.

bug:18640307
Change-Id: I2238eabaf665cad0a4f4258add57611f2b450089
diff --git a/services/core/java/com/android/server/connectivity/Vpn.java b/services/core/java/com/android/server/connectivity/Vpn.java
index f08a652..bd0e587 100644
--- a/services/core/java/com/android/server/connectivity/Vpn.java
+++ b/services/core/java/com/android/server/connectivity/Vpn.java
@@ -727,6 +727,7 @@
                 if (interfaze.equals(mInterface) && jniCheck(interfaze) == 0) {
                     mStatusIntent = null;
                     mVpnUsers = null;
+                    mConfig = null;
                     mInterface = null;
                     if (mConnection != null) {
                         mContext.unbindService(mConnection);