Framework changes for 464xlat.

1. Add a Nat464Xlat service that ConnectivityService can use
   to start and stop clat. When clat is started, the service
   waits for the clat interface to come up and then calls
   ConnectivityService to add the appropriate routes.
2. Make ConnectivityService start clat when an IPv6-only mobile
   interface is connected. We only support clat on mobile for
   now.
3. Make tethering use the interface that has the IPv4 default
   route insted of using the base interface of the
   LinkProperties. This allows us to tether to a stacked
   interface, which is needed for tethering with 464xlat.

Bug: 8276725
Change-Id: I24480af69ee280f504399062638af0836a56268e