Add active wallet count P3A metrics

[DJAndries]

Resolves brave/brave-browser#25504

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:

0. Start browser with fresh profile and --p3a-count-wallet-test-networks command line switch.
1. Ensure metrics do not exist on local state page.
2. Create new wallet, create various accounts for Ethereum, Solana and Filecoin. If on Android, close and re-open wallet, wait 10 seconds.
3. Ensure metrics do not exist on local state page.
4. Deposit various testnet native coins and chain tokens (such as ERC20, ERC721 and SPL tokens) into the accounts. Acquire coins/tokens via a testnet faucet.
5. Reload the wallet page, unlock if necessary. If on Android, wait 10 seconds.
6. Check local state page, ensure metrics contain expected values.
7. Start browser without --p3a-count-wallet-test-networks switch and open/unlock wallet. If on Android, wait 10 seconds.
8. Ensure metrics contain a zero value.