Support adding NATT keepalive packet filter

Support adding NATT keepalive packet filter to APF
filter.
Generating APF program will be addressed in another CL.

Bug: 33530442
Test: - atest NetworkStackTests
      - atest FrameworksNetTests

Change-Id: I403cd14ac9aa6b001c4e580abbb33a615931a192
Merged-In: Idaa7238a5c9acdae9f6cff13095ee9436c7c92c8
(cherry picked from commit 038c11d5)