Implement AccountsMutator::MoveAccount() method to replace PO2TS::ExtractCredentials()

This method implements the AccountsMutator::MoveAccount API as a wrapper to the
existing PO2TS::ExtractCredentials API.

Note that the implementation body also calls AccountTrackerService::SeedAccountInfo,
saving the caller site to do so (an example of its usage can be seen in [1]).

[1] https://crrev.com/c/1426457

BUG=922482

Change-Id: I63ef547dab7813a8dec99ec071a65d461ab6912d
Reviewed-on: https://chromium-review.googlesource.com/c/1425663
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624780}
6 files changed