GitLab

Refactored assistant availability check to a central location.
Also added a notification when policy is disabled.

BUG=b:69167025
TEST=run with test domain account, see entry points disabled

Change-Id: I18a3b47c4ffa8ce8cb3d18eb30315cbfec83f16d
Reviewed-on: https://chromium-review.googlesource.com/776117
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517556}