Close #26320: Make Nimbus first-startup comments clear in FenixApplication

[jonalmeida]

Adding a bit of cleanup that makes it clear we don't want anyone to add things to the application onCreate before Nimbus. No logical changes; cleanup.

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.

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• QA Needed

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GitHub Automation

Fixes #26320

[csadilek]

👍 ✨

[jonalmeida]

@jhugman added you as a reviewer so you're aware.