[sync] DualLayerUserPrefStore: update getters/setters for preexisting

This CL updates the getters and setters to take care of account pref
values loaded from the persistent storage before the sync machinery
actually starts.

The getters can now read the account store values, even
before the sync starts, but setters still do require the type to be
active.

Bug: 1447588
Change-Id: Ia92acdba24b7acbbd34d9f628213130d7b335301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4484543


Commit-Queue: Ankush Singh <ankushkush@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147350}