Refactor Brave Ads served events

[tmancey]

Resolves brave/brave-browser#27942

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:

Confirm ads per hour and ads per day frequency caps are working for all creative ad types. Confirm Brave News and New Tab Page ads are working as expected when Brave Ads is disabled.

[github-actions]

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[github-actions]

⚠️ PR head is an unsigned commit
commit: 80365c3dde2520cc6d9ffa332d2ba97e0cfd0fdb
reason: unsigned
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cc: @tmancey

[github-actions]

⚠️ PR head is an unsigned commit
commit: 80365c3dde2520cc6d9ffa332d2ba97e0cfd0fdb
reason: unsigned
Please follow the handbook to configure commit signing
cc: @tmancey

[github-actions]

⚠️ PR head is an unsigned commit
commit: 69ed1b726869e569d60cba4efd6875377c7f3e29
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cc: @tmancey

[github-actions]

⚠️ PR head is an unsigned commit
commit: b544f2b55ce6bf8aaf2a901f74a158fa49c59d98
reason: unsigned
Please follow the handbook to configure commit signing
cc: @tmancey