Move the iptables metadata marking rule earlier in
router init, that way any stray metadata requests
that arrive before the filter metadata redirect rule is
installed will just be dropped. We do this irregardless
of whether we will be running the metadata proxy.
Partial-bug: #1735724
Change-Id: I8982523dbb94a7c5b8a4db88a196fabc4dd2873f
(cherry picked from commit 6941977827)