Make a v4 nattKeepalivePacket helper method am: df347448a1 am: 8802197935

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2616794

Change-Id: If797d5ac4dfc926ca666636adf6ee9cdecd77985
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>