Rename repeated proto fields to use plural form

This CL renames all repeated proto fields in
device_management_backend.proto to use plural form.
(See https://cloud.google.com/apis/design/naming_convention#repeated_field_names)

Bug: none
Change-Id: I91d623910943c5d3faa9c38313fd66a9f39a6d47
Reviewed-on: https://chromium-review.googlesource.com/c/1470232
Commit-Queue: Alexander Hendrich <hendrich@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632227}
22 files changed