Limit the translation code on NAT-T keepalive

Limit the translation code on NAT-T keepalive because the
intention of the translation is design for NAT-T keepalive.
The translated packets is also NAT-T packet data which should
not apply to TCP keepalive.

Bug: 196453719
Test: atest FrameworksNetTests
Change-Id: I0951621edc9f918199d82365a6f1c03617ddd806