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/cryptohome.py b/client/cros/cryptohome.py
index a0c6734..8a6b567 100644
--- a/client/cros/cryptohome.py
+++ b/client/cros/cryptohome.py
@@ -5,9 +5,10 @@
 import dbus, gobject, logging, os, random, re, shutil, string
 from dbus.mainloop.glib import DBusGMainLoop
 
-import common, constants
+import common
 from autotest_lib.client.bin import utils
 from autotest_lib.client.common_lib import error
+from autotest_lib.client.common_lib.cros import constants
 from autotest_lib.client.cros.cros_disks import DBusClient
 
 CRYPTOHOME_CMD = '/usr/sbin/cryptohome'