[Extensions] Make ExtensionPrefsHelper a keyed service.

This is another step in disentangling the Preference API, the
FontSettings API, and the Settings Overrides API. It moves some helper functions from the PreferencesAPI class to a keyed service in
/extensions/browser.

This is a reland of an earlier CL with some important changes:

1. The PreferenceAPI is not created in unit tests, but in the original version of this CL, the ExtensionPrefsHelper instance was created. This led to flakiness on Mac builds. This CL follows the original pattern for the PreferenceAPI so the ExtensionPrefsHelper class is not created in unit tests.

2. In the original CL, there were changes to a ExtensionMessageBubbleTest unit test to populate an underlying data structure that the ExtensionPrefsHelper relies on. Since it's no longer being created in unit tests, the changes to populate that data structure are removed.

The original CL, which was reverted, is here:

commit 32fe4701ea9b87e3f7d0a51a99cb0a202a1273b8.

With the original CL, ExtensionMessageBubbleTest.SettingsApiControllerTest was quite flaky on the mac-rel bot. With this CL, the test passed 3000 iterations without any flakes.

LOW_COVERAGE_REASON=Seems related to recent code added for Lacros.

Bug: 883570
Change-Id: I85e4919360bbf6660f022ba161e453ebf1ffe814
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3988363
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067616}