When used as part of presubmit's input_api, we only
expose OwnersClient and not global constants, so
move constants inside OwnersClient.

Change-Id: I8983c5edadafeebff62838d9cd4a19fdce908159
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2621744
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>