Stop clatd when starting the Nat464Xlat service.

If a runtime restart happens while clatd was running, we try to
start clatd, which causes a fatal exception because netd returns
a 400 error (clatd already started.

Bug: 13450716
Bug: 15012035
Change-Id: I102a06d6193fb5f4a1ebe5ad52e5647ff72ca0da