Make a v4 nattKeepalivePacket helper method am: df347448a1

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

Change-Id: I93318e497128a74410367233b4263f2b5f85e164
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>