Reland "Changing OAuth2TokenService::Observer and its subclasses to use CoreAccountId instead of std::string"

This is a reland of 2669b5b556b8d0c7d0915a6cd3cbb1f597489999

The previous cl had a merge conflict with another cl that introduced a new file
components/signin/core/browser/oauth2_token_service_delegate_android_unittest.cc

This cl fixes the error.
> Changing OAuth2TokenService::Observer and its subclasses to use CoreAccountId instead of std::string
>
> This cl is part of the effort to use CoreAccountId instead of std::string
> to denote account_id.
>
> Bug: 959157
> Change-Id: I6b95b6adf7d73228c399162daf2867c532acd7cd
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626599
> Reviewed-by: David Roger <droger@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
> Cr-Commit-Position: refs/heads/master@{#663069}

Bug: 959157
Change-Id: I064945cb25872e8c87e3f9012cd4cf44a9c98d7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628622
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#666220}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4fcc63ae63dfcc04f42d9d8ca750f7ab9e39a26e
3 files changed