[NFCT.NS.3] Add netfilter conntrack event monitor

A preparation for starting monitoring conntrack event. Only the
netlink message header has been parsed for now. Need follow-up
commits to parse the netfilter message header and conntrack
attributes.

Test: atest TetheringCoverageTest
Change-Id: I71ca6cf75206fe8da7b5bb775b8debee6ab996b4