libbrillo: policy: Add new update time restrictions policy

Add a new device policy from chrome_device_policy.proto:
DeviceAutoUpdateTimeRestrictions

BUG=chromium:852860
TEST=libpolicy unit tests
CQ-DEPEND=CL:1136538
Change-Id: Iaef8791f683af00668dcd4041282758cc26b8fbf
Reviewed-on: https://chromium-review.googlesource.com/1101707
Commit-Ready: Adolfo Higueros <adokar@google.com>
Tested-by: Adolfo Higueros <adokar@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
6 files changed