Support NAT keepalives

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
    - turned on debugging and walked around watching IpReachabilityMonitor
      force neighbors into NUD_PROBE state without error
    - observed NAT callbacks happening without any reported errors
    - watched:
        adb shell cat /proc/net/nf_conntrack | egrep '192[.]168[.]43[.]' | sort -n -k5
      for correct timeout updates

Bug: 29337859
Bug: 32163131

Change-Id: Icb23da64cfaa3a19f7bc75fba426a52b0994fb0f