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  - Loads Login Scoped Tokens (LSTs) from disk to memory
  - Persists updates to LSTs to disk

Design doc is linked in the bug id.
This CL creates the core of Account Manager and its ability to load
and persist its state.
Follow up CLs will:
- Link it with OAuth2TokenService via an OAuth2TokenServiceDelegate.
- Add UI components to modify Account Manager's state.

Bug: 820046
Change-Id: I1b43e11cfc7ed3592daf4a79ff664d377644f6a8
Test: chromeos_unittests --gtest_filter="*AccountManager*"
Reviewed-on: https://chromium-review.googlesource.com/955523
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543944}