[s13n] Convert ArcAuthContext away from ProfileOAuth2TokenService

CL converts ArcAuthContext away from PO2TS, as part of the
IdentityManager servicification effort (//services/identity).
No functionality change is expected.

Note that there is still one reference to PO2TS left, that is used to
construct UbertokenFetcher. It will be removed on a follow up pass.

CL also takes the opportunity to migrate ArcBackgroundAuthCodeFetcher
away from OAuth2TokenService::Consumer, since it is tied to
ArcAuthContext.

BUG=905243

TEST=Run on test device.
TEST=Run browser_tests --gtest_filter=Arc*
TEST=Run unit_tests --gtest_filter=Arc*

Change-Id: I1a74df3b79a7753e96dc7993d636692efa18d1a0
Reviewed-on: https://chromium-review.googlesource.com/c/1345130
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610189}
4 files changed