Add TcpKeepalivePacketData to SystemApi

This is consistent with NattKeepalivePacketData, which is also a
subclass of KeepalivePacketData.

TcpKeepalivePacketData is already used by the wifi module, but
statically linked.

Bug: 172789687
Test: m
Change-Id: I6aee1ae205987521bea4a3838bbece279ffa0e37