Disable Chrome OS Account Manager for Public Sessions

Summary of changes:
  - Consolidate the logic of checking Chrome OS Account Manager's
    availability into 1 utility function.
  - Make this utility function return |false| for the availability of
    Chrome OS Account Manager in Public Sessions.

Bug: 926157
Change-Id: Ife2e5fffe95ca8c08c42376e80a5950785d4669f
Reviewed-on: https://chromium-review.googlesource.com/c/1486333
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636501}
9 files changed