Rewards publisher list is now updated every 24 hours

[jasonrsadler]

Fixes brave/brave-browser#3171

Previously was updated every 48 hours.

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) on
  • Windows
  • macOS
  • Linux
• 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

Test Plan:

1. Start Brave and enable rewards
2. Quit Brave and open publisher_state file
3. set pubs_load_timestamp to a value more than 24 hours in advance and save file
4. Restart Brave and check timestamp is updated on publishers_list file
5. Open publisher_state and ensure pubs_load_timestamp has been updated.

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