[ads] Add Sponsored Images & Videos option to New Tab Page setting

[aseren]

Sponsored Images & Videos option is currently shown for Japan region only:

Sponsored Images & Videos option is hidden for all other regions:

Resolves brave/brave-browser#38198

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• 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 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Prerequisites to install video NTT staging components

New Tab Takeover video ads are available in staging component updater for iOS.
Steps to setup staging component updater on iOS:

• Fresh install
• Launch browser
• Toggle Use Staging CRX components in Settings -> BraveCore Switches
• Close the browser
• Join Japan VPN
• Launch Brave browser
• Join Brave Rewards
• Staging components are downloaded and applied in a minute after start

Test case 1 (Preferences migration check)

• Set your device region to something other than Japan
• Install the previous version of Brave (version 1.70)
• Make sure that setting Background Images -> Image Type is set to Sponsored
• Install the latest version of Brave (version 1.71 and higher)
• Make sure that setting Background -> Media Type is set to Sponsored Images
• Make sure Sponsored Images are shown on NTP
• Set setting Background -> Media Type to Default Images
• Make sure that Sponsored Images aren't shown on NTP

Test case 2 (Preferences migration check)

• Set your device region to something other than Japan
• Install the previous version of Brave (version 1.70)
• Set setting Background Images -> Image Type to Default Images
• Install the latest version of Brave (version 1.71 and higher)
• Make sure that setting Background -> Media Type is set to Default Images
• Make sure that Sponsored Images aren't shown on NTP
• Set setting Background -> Media Type to Sponsored Images
• Make sure Sponsored Images are shown on NTP

Test case 3 (Preferences migration check)

• Set device region to Japan
• Install the previous version of Brave (version 1.70)
• Make sure that setting Background Images -> Image Type is set to Sponsored
• Install the latest version of Brave (version 1.71 and higher)
• Make sure that setting Background -> Media Type is set to Sponsored Images & Videos
• Make sure that Sponsored Images (or Sponsored Videos) are shown on NTP

Test case 4 (Fresh install)

• Set your device region to something other than Japan
• Install the latest version of Brave (version 1.71 and higher)
• Make sure that setting Background -> Media Type is set to Sponsored Images
• Make sure Sponsored Images are shown on NTP
• Set setting Background -> Media Type to Default Images
• Make sure that Sponsored Images aren't shown on NTP

Test case 5 (Fresh install)

• Set device region to Japan
• Install the latest version of Brave (version 1.71 and higher)
• Do prerequisites to install video NTT staging components
• Make sure that setting Background -> Media Type is set to Sponsored Images & Videos
• Make sure that Sponsored Videos are shown on NTP
• Make sure that Sponsored Images are shown on NTP
• Set setting Background -> Media Type to Sponsored Images
• Make sure that Sponsored Videos are not shown on NTP
• Make sure that Sponsored Images are shown on NTP

[brave-builds]

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