SyncPrefs: Change APIs from GaiaIdHash to GaiaId

Some of the APIs require the actual Gaia ID, not just a hash. This
change makes the overall API more consistent, and as a side benefit,
many call sites becomes slightly nicer since they don't have to do the
hashing anymore.

Fixed: 399124613
Change-Id: I56597e3279934dc0e77248ad72dd5ba3fb5620ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6535752
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Maria Kazinova <kazinova@google.com>
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1459976}