cryptohome: support key policies and LE credentials

This CL adds new functionality to cryptohome API:
1) Specifying policies for keys.
2) Requesting info on the supported key policies.
3) Key policy for low entropy credentials.

BUG=chromium:794010
TEST=emerge system_api

Change-Id: I86f111291366fa6b88632b8ba919183b644fc76b
Reviewed-on: https://chromium-review.googlesource.com/448976


Commit-Ready: Andrey Pronin <apronin@chromium.org>
Tested-by: Andrey Pronin <apronin@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>