Send message to add/remove NAT-T keepalive packet filter.

Remove definition of TYPE_NATT and TYPE_TCP since the type
can be identified by checking message.obj is an instance of
NattKeepalivePacketData or TcpKeepalivePacketData.
It's more simple and won't have dependency on KeepaliveInfo.

Bug: 33530442
Test: atest FrameworksNetTests
      atest NetworkStackTests

Change-Id: Ic97ffe9ff5781778efd264460809f5059f0f4230