Fix Ads enabled when joining Rewards via panel

[ryanml]

Fixes: brave/brave-browser#4156

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests && npm run test-security) on
  • Windows
  • macOS
  • Linux
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection

Test Plan:

There are different testing permutations to ensure that Ads status is correct.
[@tmancey and myself ran through these and verified]

Fresh install is creating a profile with the version in this PR, Upgrade means upgrading from a version <0.62.x

Rewards never enabled

1. PASSED
   Rewards enabled set to false
   Ads enabled set to false
   Upgrade

2. PASSED
   Rewards enabled set to false
   Ads enabled set to false
   Fresh install

Via Rewards Panel

1. IMPOSSIBLE SCENARIO
   Rewards enabled set to false
   Ads enabled set to true
   Upgrade

2. PASSED
   Rewards enabled set to true
   Ads enabled set to false
   Upgrade

3. IMPOSSIBLE SCENARIO
   Rewards enabled set to true
   Ads enabled set to true
   Upgrade

4. IMPOSSIBLE SCENARIO
   Rewards enabled set to false
   Ads enabled set to true
   Fresh install

5. IMPOSSIBLE SCENARIO
   Rewards enabled set to true
   Ads enabled set to false
   Fresh install

6. PASSED
   Rewards enabled set to true
   Ads enabled set to true
   Fresh install

Via brave://rewards

1. IMPOSSIBLE SCENARIO
   Rewards enabled set to false
   Ads enabled set to true
   Upgrade

2. PASSED
   Rewards enabled set to true
   Ads enabled set to false
   Upgrade

3. IMPOSSIBLE SCENARIO
   Rewards enabled set to true
   Ads enabled set to true
   Upgrade

4. IMPOSSIBLE SCENARIO
   Rewards enabled set to false
   Ads enabled set to true
   Fresh install

5. IMPOSSIBLE SCENARIO
   Rewards enabled set to true
   Ads enabled set to false
   Fresh install

6. PASSED
   Rewards enabled set to true
   Ads enabled set to true
   Fresh install

Restart Browser

1. PASSED
   Rewards enabled set to false
   Ads enabled set to false

2. IMPOSSIBLE SCENARIO
   Rewards enabled set to false
   Ads enabled set to true

3. PASSED
   Rewards enabled set to true
   Ads enabled set to false

4. PASSED
   Rewards enabled set to true
   Ads enabled set to true

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[tmancey]

LGTM and worked with @ryanml to create the test plan to cover all permutations