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Tests failed because assistant was disallowed by account type for the
following reasons:
- Autotest with real gaia login:
  Caused by a race between chromeos login finished and account info fetch
  finished.
- Tast with fake gaia login:
  Account info fetch not be faked under fake gaia env.

This changes fixes these issues by first retry to initiate assistant when
account info updated, and also bypass the account type check in
assistant::IsAssistantAllowedForProfile under fake gaia env.

Misc:
Add IsGaiaServicesDisabled helper function in chromeos_swithes.

Bug: 942781, 939146, 940618
Test: local compile and run autotest/tast tests on DUT.
Change-Id: I25bdcda9587988426745c2dd036da1b8be6f636a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1530119
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642370}