Support more than one clatd at a time.

1. Make Nat464Xlat a per-network object, one for every network
   requiring clat, instead of a ConnectivityService singleton.
2. Make the NetworkManagementService clatd commands take an
   interface.
3. When we attempt to start clatd on a network, store its
   Nat464Xlat object in the NetworkAgentInfo, so we have an
   authoritative way of knowing whether clat is running on a
   given network.
4. Rework Nat464Xlat, hopefully simplifying it.

Bug: 12111730
Change-Id: I1fa5508ef020cd1c3d1c7a1f7b06370ac5fc2ae2