Unbreak interface add/delete notifications.

The change to enable address tracking via netlink incorrectly
changed the subsystem of rtnetlink events from "net" to
"interface". This broke interface add/delete notifications,
which come from the kernel with subsystem "net".

Switch back to "net" and deal with address tracking via new
action codes instead of a new subsystem.

Bug: 10433320
Change-Id: I59a50e9c7cb49f46e680c7d84ac8e196a861ca4b