Add support for running 464xlat on wifi as well.

1. Add a command to NetworkManagementService to enable/disable
   IPv6 ND offload via netd.
2. Make Nat464Xlat enable offload if clatd successfully comes up
   on a wifi network (which means it detected a NAT64), and
   correspondingly re-enable offload when the clatd interface
   goes down.

This change does not enable clatd on wifi yet, that requires an
extra 2 lines to enable it.

Bug: 12111730
Change-Id: I4318611762a37487c9a84f8c4867ec5aece98be8