Closes #26446: add wallpaper oboarding Nimbus flag

[mavduevskiy]

Adding a nimbus flag for wallpaper onbarding.

Pull Request checklist

• [ ] Tests: This PR includes thorough tests or an explanation of why it does not
• [ ] Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• [ ] Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

QA

• [x] QA Needed

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GitHub Automation

Fixes #26446