Add helper method to generate a v6 NAT-T keepalive packet

This is a part of preliminary work for sending a v6 NAT-T
keepalive packet in a v6 only network. This commit
release the constraint on constructing a v6 NAT-T keepalive
packet.

Bug: 196453719
Test: Manually connect to v6 only network and check if the
      keepalive packets are sent with subsequent commit
Test: atest FrameworksNetTests with the updated test
Change-Id: I9dc96e840265c5dee6b9e3ef2b3901a0aeb4ff9e
3 files changed