libbrillo: policy: Modify UpdateStaging policy due to change in design

Modify the DeviceUpdateStagingSchedule policy code to account for the
changes to use JSON strings. Add JSONToListValue to remove repeated
logic between new JSON based policies.

BUG=chromium:858621
TEST=cros_workon_make libbrillo --test
CQ-DEPEND=CL:1158225

Change-Id: I01f73980c5309d77bfe04861255d6665cb7e2e11
Reviewed-on: https://chromium-review.googlesource.com/1142480
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Tested-by: Adolfo Higueros <adokar@google.com>
Reviewed-by: Dan Erat <derat@chromium.org>
5 files changed