Fix findConnectionTypeForIface.

This is used by the VPN to notice when its underlying connection
has gone away. I'm fixing this using network types and not
NetworkCapabilities because

1. I don't know of a way to use the new API to get callbacks for
   a specific network. You can get them based on capabilities,
   but it's not clear how to construct a NetworkCapabilities
   object that will only match a specific network, or only match
   the default network.
2. It's a smaller change.

Bug: 15070628
Change-Id: Id6a6a183da83636c0859db4c954093bd684c01ea