Nat464Xlat: rely on netd events being called on handler thread.

aosp/2724918 moved processing netd events for 464xlat to the
handler thread.

This CL makes 464xlat run the code that processes those events
inline. This simplifies the code and makes the ordering of events
more similar to what it was before.

Bug: 293965195
Fix: 302071735
Test: existing unit tests
Change-Id: I18b0d491aff94646b878a3d3488b5519fd42783c
4 files changed