FakeSessionManagerClient: Refactor policy storage code

Cleans up policy storage code in FakeSessionManagerClient (for the
USE_HOST_POLICY case), highlighting common grounds with the non-stub
SessionManagerClient code and making it more easily extensible. No
functional changes.

BUG=chromium:765644
TEST=Tryjobs

Change-Id: If31dbaf8d17292539c581f23306c3eaf0a6bf483
Reviewed-on: https://chromium-review.googlesource.com/972381


Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546880}