Make the VPN notice connectivity changes on stacked interfaces.

Currently, the VPN code only looks at base interfaces, so if
464xlat is in use, it will never disconnect when its underlying
network has disconnected. Use getAllInterfaceNames().contains()
instead of just comparing the base interface name.

Bug: 19336810
Change-Id: Id1ba2c80a09cec5098c6f512acdc0a05a939e9f3