libbrillo: policy: Increase resilience for policy read

The policy files will be stored with indexes as suffix to improve
resilience to disk corruptions. This change makes it possible to
read the policy data from new files and validate them.

Design doc:
https://docs.google.com/document/d/1RdUKb-deQCBiyc6Dpdf4RDBEPV-feb5WHu7uzY1YozQ/edit#heading=h.h0q5njx6l4xf

BUG=chromium:764337
TEST=Manual
CQ-DEPEND=CL:681939, CL:819250, CL:819353, CL:819412

Change-Id: I10f31d307925c6ccef3cfd0887a71b7c774139e2
Reviewed-on: https://chromium-review.googlesource.com/674935


Commit-Ready: Igor <igorcov@chromium.org>
Tested-by: Igor <igorcov@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
(cherry picked from commit d213a401)
Merged-In: I10f31d307925c6ccef3cfd0887a71b7c774139e2