Ensure netlink messages come from the kernel.

Currently clatd accepts all netlink messages without validating
that they come from the kernel. This could allow another app to
spoof these messages. Fix this by connecting the socket to the
kernel so that no other process can send it messages.

Bug: 7664960
Change-Id: I994641ea13cfd07fb25ccf52fcbbf5d1c9633ec4