Fix initial Rewards state migration for old versions

[zenparsing]

Resolves brave/brave-browser#16449

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:

Scenario: Reading from legacy JSON files on upgrade from old version

• Given that the user profile directory contains ledger_state and publisher_state JSON files
• And the brave.rewards.version profile preference does not exist
• When the user restarts the browser
• Then the user's Rewards wallet information should be imported from the ledger_state file

Possible test steps

• Start the browser with a clean profile to create a profile directory.
• Close the browser.
• Copy ledger_state and publisher_state JSON files into the profile directory.
• Start the browser.
• Navigate to brave://rewards-internals.
• Verify that the displayed payment ID matches the payment ID from the ledger_state file.

[zenparsing]

Putting this back into draft, as it causes issues with first-time startup.

[emerick]

LGTM