[signin] SigninErrorController observes the TokenService

To simplify authentication error handling, this CL updates the
SigninErrorController to get its error state from the token service by
observing it.
This removes some coupling between the classes, and removes the
AuthStatusProvider mechanism, leading to an overall simplification of the code.

TBR=eugenebut

Bug: 836212
Change-Id: Ie9b849fcd804d4fe6b4787b277b27fb848cdf2dc
Reviewed-on: https://chromium-review.googlesource.com/c/1070154
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612610}