Refactor PromoService to use PrefService::GetValueDict

This is part of a larger migration effort based on
go/trust-your-prefs.

Bug: 1341996
Test: Existing unit tests
Change-Id: I7ca0ad05aa69f485c12d8907640214c4982e6dc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3829271


Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Roland Bock <rbock@google.com>
Cr-Commit-Position: refs/heads/main@{#1040901}