Nat464Xlat: interface notification handler on ConnectivityService

This patch adds a layer of asynchonicity to the NetworkBaseObserver
callbacks implemented by Nat464Xlat in order to allow these callbacks
to run on the main ConnectivityService handler.

This allows to run interfaceLinkStateChanged and interfaceRemoved
callbacks in the same thread context as other Nat464Xlat methods and
solves the following issues:
  - NPE risk due to race between fixupLinkProperties called on the
    ConnectivityService thread and interfaceRemoved called as a
    callback by NetworkManagementService.
  - stale LinkProperties reads in both callbacks not called on
    ConnectivityService handler.
  - removes the race between stop() and interfaceRemoved().

This patch also:
  - removes/simplifies comments related to the threading
    model which are no obsolete.
  - extract clatd management logic from ConnectivityService into
    NetworkAgentInfo

Bug: 62997041
Bug: 64571917
Test:  runtest frameworks-net
       manually connected to ipv6 network and went to test-ipv6.com
Change-Id: I889d98e47423ff3d4746d6ed8015b265286e7c52