Use {To/From}JavaTime for policy timestamps

Use base::Time::ToJavaTime() and base::Time::FromJavaTime() for
conversions between base::Time and timestamps in the policy protos.

No change of behavior should be introduced, it's just a refactoring.

BUG=701045
TEST=existing tests

Review-Url: https://codereview.chromium.org/2830033003
Cr-Commit-Position: refs/heads/master@{#466325}
15 files changed