Translate v4 keepalive packet on clat started network to v6

This commit translates the address of keepalive packet to v6
in order to make NATT keepalive work on v6 only network.

Sending a v4 NATT keepalive on a v6 only network is bogus if
hardware does not translate the packet automatically. Network
may drop the packet or reject the attempt by hardware. This
results in ERROR_INVALID_IP_ADDRESS and stop the keepalive
request.

Bug: 196453719
Test: atest FrameworksNetTests
Test: Manually connect to v6 only network and check if the
      keepalive packets are sent
Change-Id: I798c5d48661d8151b19902435732f465b5aa725b