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/chrome_binary_test.py b/client/cros/chrome_binary_test.py
index ac81238..18866f5 100755
--- a/client/cros/chrome_binary_test.py
+++ b/client/cros/chrome_binary_test.py
@@ -6,7 +6,7 @@
 
 from autotest_lib.client.bin import test, utils
 from autotest_lib.client.common_lib import error
-from autotest_lib.client.cros import constants
+from autotest_lib.client.common_lib.cros import constants
 from autotest_lib.client.cros.graphics import graphics_utils