libpolicy: Add getter for ChromeDeviceSettingsProto

Adds a getter for ChromeDeviceSettingsProto in DevicePolicyImpl. This
prevents a lot of boilerplate code when used by classes that deal with
device policy, anyway, like authpolicyd.

Also adds a way to disable validation to be used for authpolicyd unit
tests.

BUG=chromium:801704
TEST=cros_run_unit_tests --board=amd64-generic --packages libbrillo

Change-Id: Ide9e8f87c9798c6d482e9d88bb7e2f909b1ff02e
Reviewed-on: https://chromium-review.googlesource.com/893180
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
4 files changed