[Brave News] Fix first open performance of the Customize UI

[fallaciousreasoning]

Resolves brave/brave-browser#26197

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. Enable Brave News V2
2. Open the Brave News Customize dialog

Previous versions (at least on low end devices) had a sold 0.5 - 2s of jank before the dialog became interactive.

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[sangwoo108]

LGTM 👍 Insightful!

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[stephendonner]

Verified PASSED using

Brave	1.47.20 Chromium: 107.0.5304.68 (Official Build) nightly (x86_64)
Revision	a4e93e89d3b3df1be22214603fba846ad0183ca5-refs/branch-heads/5304@{#991}
OS	macOS Version 11.7.1 (Build 20G918)

My highly unscientific tests show about 1-2 seconds worth of time saved when opening the Brave News Customize dashboard for the 1st time.

Steps:

1. installed each respective (pre- and post-fix) build
2. launched Brave
3. opened brave://flags
4. toggled brave://flags/#brave-news-v2 to Enabled
5. clicked Relaunch
6. open a new-tab page
7. click on Customize
8. get a stopwatch started
9. click on Turn on Brave News at the 2-seconds mark
10. click to stop the stopwatch once the Customize dashboard's dialog is fully rendered

1.47.9	1.47.20