Nat464Xlat: clat management cleanup

This patch does some minor refactoring of clat starting/stopping code:
 - remove unused LinkProperties arguments in updateClat
 - remove unused Context argument in Nat464Xlat ctor
 - introduce ensureClatIsStarted and ensureClatIsStopped methods and
   simplify updateClat
 - add clatd to NetworkAgentInfo toString() method
 - clarify some comments

This changes prepare for moving BaseNetworkObserver callbacks to
ConnectivityService.

Bug: 62997041
Bug: 64571917
Test: runtest frameworks-net
      manually connected to IPv6 only network and went to test-ipv6.com

Merged-In: Idb204784614cfe700f73255a7a7b78c5e9ee6eca
Merged-In: Ic3808a1afe48efac745b1b378fb12cc5678918ec
Merged-In: Ia769aef6ef8b258f44f8979003d271c96264f1b5
Merged-In: I1a19e6fbb0cb13262e90b171d861062469078fb6
Merged-In: I06661bd6bd1456ba34a3bbdb52c120ac01da9d61
Merged-In: Ifccff9f3cfccdb2cdddf2f07561f0787a48bc0f8

(cherry picked from commit b577d658)

Change-Id: Ibb02888633df9643030336c4dbea6c569a47554c