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
Merged-In: I6efc9fed2420ca488731a2b9b9c3c025b16eca10
Merged-In: I188ac4c367db11cb33b67fe92df3a120e3c6fbce
Merged-In: I7e2c5db8d537fb0ab47cde37158b7f04d7786942
Merged-In: Ic2246a97618c596dbdbf48cda39c2f5b66e3bfb6
Merged-In: Ib04b9a3d56e9daf61b299a30e24a3c8839819a00
Merged-In: Icc1558a0f0e7c299270f550897347458e2bd3188

(cherry pick from commit 4f6f1398)

Change-Id: Iebc7d153d8cd0b90d074d8d6eed821fbc3f1370d