[Identity Service] Move from using AccountId to using AccountInfo

Initially it seemed like a good idea to use AccountId, since that was
an existing Mojo type. However, that type was created for authentication
in ARC++ and is specific to that use case. It makes more sense for the
Identity Service to talk in terms of AccountInfo, the mainline struct
for account information that the core signin code uses. In the long
term we'll flesh out what the right relationship between AccountId and
AccountInfo should be.

[email protected]

Bug: 683124
Change-Id: I150c59762a370a9a209dc92a215a19ae1c8c953f
Reviewed-on: https://chromium-review.googlesource.com/517490
Commit-Queue: Colin Blundell <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Mihai Sardarescu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#478946}
9 files changed