Refactor wallet importer

[darkdh]

Resolves brave/brave-browser#18534
Resolves brave/brave-browser#18570

1. ExternalWalletsImporter is dedicated for checking external wallet installation, initialization and import data from it.
2. Mojo API now has only IsExternalWalletInstalled, IsExternalWalletInitialized and ImportFromExternalWallet with enum wallet type.
3. Accept trailing comma in importer as a precaution

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
• 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:

MetaMask

1. Install MetaMask and don't set it up yet
2. Navigate to brave://wallet/, there won't be Import from MetaMask button available
3. Finish setup on MetaMask
4. Navigate to brave://wallet/, now it shows Import from MetaMask button
5. Finish import process and check seeds and accounts are correct.

Crypto Wallets

1. Navigate to brave://flags/#native-brave-wallet and choose Disabled and relaunch
2. Navigate to brave://wallet/ and click "I Understand" but don't create/restore wallet
3. Navigate to brave://flags/#native-brave-wallet and choose Default and relaunch
4. Navigate to brave://wallet/, it won't show previous wallet detect message
5. Repeat step 1 and setup Crypto Wallets and repeat step 3
6. Navigate to brave://wallet/, it now shows Existing crypto wallets detected
7. Finish import process and check seeds and accounts are correct.

[bbondy]

Nice cleanup :)