Wait for initial rewards API parameter fetch from server

[zenparsing]

Resolves brave/brave-browser#10438

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
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• 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-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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

See brave/brave-browser#10438 for original test cases.

Test Case: AC amount drop down is fully populated after enabling rewards

• Launch with a clean profile, pointed to staging.
• Navigate to brave://rewards.
• Enable rewards.
• Scroll down to AC settings.
  • Verify that a valid amount is display in the AC amount selection.
• Click the AC amount drop down arrow.
  • Verify that a list of valid selections are displayed.

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.

[zenparsing]

Updated PR so that callbacks are always run with prefs data, even if the fetch fails for some reason.

[LaurenWags]

While testing on Nightly, issues were encountered that are most likely specific to code changes in 1.12.x and 1.13.x. Since the fix for this is intended for 1.11.x, this was tested with a PR build and notes can be found here: #6050 (comment)

A follow up issue was logged to address remaining 1.11.x issues as well as problems seen while testing on Nightly: brave/brave-browser#10717