cros: Make policy test server more configurable after it started

- Reloads client_state file every time
- Added possibility to save state keys for clients
- LocalPolicyTestServer provides function to store configuration file

I'm gonna introduce LocalPolicyTestServerMixin in the next CL

Bug: 934224
Change-Id: I7960159b5db8c62e2515f8cedccd469e66a4192e
Reviewed-on: https://chromium-review.googlesource.com/c/1482459
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636061}
9 files changed