Nat464Xlat: internal state guards cleanup + state enum

This patch does some cleanup of Nat464Xlat internal state guards
against the Nat464Xlat state Idle | Started | Running, which reduces
code nesting.

It also replaces introspection of internal state for distinguishing
between different stages in 464xlat lifecycle with an enum explicitly
introducing these three Idle | Started | Running states.

Bug: 62997041
Bug: 64571917
Test:  runtest frameworks-net
       manually connected to ipv6 network and went to test-ipv6.com
Change-Id: I6efc9fed2420ca488731a2b9b9c3c025b16eca10