apf: Remove IPv6 NAT-T keepalive related codes and ApfTest improvement

Remove IPv6 related codes since we don't support IPv6 NAT-T keepalive
as the kernel doesn't support IPv6 UDP encapsulation.

Renaming keepaliveAck to keepaliveResponce since NAT-T keeaplive
response is not an ack. Also, add generateV4NattKeepaliveFilters()
if multicast filter is disabled.

Verify incoming packet contains 1 byte payload but it is not 0xff will
pass NAT-T keepalive filter.

Bug: 33530442
Test: atest FrameworksNetTests
      atest NetworkStackTests

Change-Id: Ifb9e088c0c8d71c763ebd9ad122103d0f97ca278