Refactor unit tests to use PrefService::GetValueDict

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

Bug: 1341998
Test: Refactored unit tests
Change-Id: I4f30b54c44bbe9a4e83b2af66dd1a7f022382d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3779751
Commit-Queue: Roland Bock <rbock@google.com>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1028582}