libbrillo: Add new policy (DeviceUpdateStagingSchedule) for staging updates.

Add a new device policy that will allow administrators to specify a schedule
for rolling out updates to an organizational unit.

BUG=chromium:858621
TEST=Used YAPS as DM server, verified that the new policy is parsed correctly.
CQ-DEPEND=CL:1119162

Change-Id: I72852ddf86e6c7e432aea38a33c6fb3be4c7f315
Reviewed-on: https://chromium-review.googlesource.com/1123683


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: May Lippert <maybelle@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>