Settings: Fix FC issue when add VPN.

The cause is NullPointerException. Two vpn dialog will be shown
when double click the + icon to add vpn. The parameter mDialog
is set to null after clicking save button. Then
NullPointerException exists when click the save button in the
second vpn dialog.

Vpn dialog is shown only when the parameter mDialog is null, then
there is only one vpn dialog at the same time.

CRs-Fixed: 800473
Change-Id: I8161278adf740a8e46f54713e6847adb1cff623e