autotest: move constants to common_lib

This file gets imported from lots of server-side tests and
infrastructure so it shouldn't be hiding in client/cros. This
moves the file and fixes all imports.

BUG=chromium:485834
TEST=run many tests, make sure there are no problems importing
  constants

Change-Id: I5c2b2a0bef6ebc998e62f40162a6ec9cc4436480
Reviewed-on: https://chromium-review.googlesource.com/291302
Trybot-Ready: Eric Caruso <ejcaruso@chromium.org>
Commit-Queue: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Chris Sosa <sosa@google.com>
diff --git a/client/cros/dns_server.py b/client/cros/dns_server.py
index 57571cb..ece6be5 100644
--- a/client/cros/dns_server.py
+++ b/client/cros/dns_server.py
@@ -2,10 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import common, constants, logging, os, socket, stat, sys, threading, time
+import common, logging, os, socket, stat, sys, threading, time
 
 from autotest_lib.client.bin import utils
 from autotest_lib.client.common_lib import error
+from autotest_lib.client.common_lib.cros import constants
 
 class LocalDns(object):
     """A wrapper around miniFakeDns that runs the server in a separate thread