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
13c9fdefde