Rename DeviceOffHoursProto fields

Rename repeated fields to plural

Bug: 739713
Change-Id: I0da1b346e52fd2590cd0a94e7a43de3f4b6d79e7
Reviewed-on: https://chromium-review.googlesource.com/640930
Commit-Queue: Daria Iakovleva <yakovleva@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Sergey Poromov <poromov@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#498423}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5b12b9454fdb7ed645a307956ce2f1ff0e507297