Make a v4 nattKeepalivePacket helper method

This is a preparation change for the subsequent changes to
separate the logic for constructing a v4 NAT-T keepalive
packets to a dedicated method.

Bug: 196453719
Test: atest FrameworksNetTests
Change-Id: If72b4875e65a547bbf90367eacce7b145358006a
1 file changed