Postpone default override warning logging until builtin preferences registered

[colin-grant-work]

What it does

Fixes #16852

Postpones logging of schema default registration warnings until all builtin preference contributions have been processed.

There could still be warnings about preferences that really do exist but that are expected to be registered by extensions and so are discovered after the registration of all builtin preferences.

How to test

1. Add "preferences": {}, before this line
2. Start the application.
3. Observe that warnings are lot logged for workbench color theme, workbench icon theme, or any actually defined preference defined in your preferences object, but are logged for any non-existent preferences referred to by the preferences config in the Electron app's package.json.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[ndoschek]

Thanks! LGTM 👍