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:

CQ-DEPEND=CL:681939, CL:819250, CL:819353, CL:819412

Change-Id: I10f31d307925c6ccef3cfd0887a71b7c774139e2
Commit-Ready: Igor <>
Tested-by: Igor <>
Reviewed-by: Maksim Ivanov <>
Reviewed-by: Dan Erat <>
(cherry picked from commit d213a401f639649b3e38f6def5782ae2f1322fce)
Merged-In: I10f31d307925c6ccef3cfd0887a71b7c774139e2
14 files changed