Fix WiFi-Direct and Tethering.

A LocalNetwork object now always exists in the NetworkController, with a fixed
NetId that's guaranteed not to collide with NetIds created by the framework.

When routes are added on an interface tracked by the LocalNetwork, they are
added to a fixed "local_network" table.

When NAT is enabled, we add a special "iif -> oif" tethering rule.

Bug: 15413694
Bug: 15413741

Change-Id: I36effc438d5ac193a77174493bf196cb68a5b97a