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This CL makes the iOS app use the newer Google account management
library instead of the deprecated one, i.e. we no longer use the side
menu and will use the circular profile button on the top-right corner
of the screen.

Code changes in this CL include:

* Introduce an AccountManager as an abstraction layer to
  Google-internal account management UI components, also with an
  AccountManagerChromium implementation for the public build.
* Deprecate AppViewController and remove AppViewControllerChromium as
  the new UI component doesn't act as the root view controller of the
  app.
* Remove ToS and Privacy Policy from the side menu items, as the new UI
  already comes with these links in the footer.

Internal change: https://chrome-internal-review.googlesource.com/c/chrome/ios_internal/+/3202666
Demo: https://screencast.googleplex.com/cast/NDYyMzk5MDAzNzQxMzg4OHxkODBmNGI4OC01YQ

Bug: 810097
Change-Id: I660e2aeef7337050e827f3a32b524deac7b0e462
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343637
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796500}