Added BR flag image to vpn panel

[simonhong]

fix brave/brave-browser#29719

Filed f/u issue (brave/brave-browser#29727) - serve flag icon from leo.

Resolves

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 wiki
  • npm run lint, npm run presubmit wiki, 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. Launch with clean profile
2. Set purchased state
3. Open vpn panel and check brazil flag image

[brave-builds]

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

[mkarolin]

GRD ++

[stephendonner]

Verified PASSED using

Brave	1.52.47 Chromium: 112.0.5615.121 (Official Build) nightly (x86_64)
Revision	39cc4e45904ae9f1741c4fbaa866e629c96f2268-refs/branch-heads/5615_51@{#8}
OS	macOS Version 11.7.6 (Build 20G1231)

Steps:

1. installed 1.52.47
2. purchased, configured, and connected to Brave VPN
3. clicked on the VPN button on the toolbar
4. confirmed flag in Brazil entry
5. clicked Connect

Confirmed the flag of Brazil appeared in both places

Dark, server list	Light, server list

Dark, connection status	Light, connection status