Fix #8109: Don't call fetch promotions on uninitialized rewards services

[kylehickinson]

Calling rewards engine APIs before the engine has been initialized can cause a crash because the WhenReady method used inside RewardsEngineImpl will queue up what is assumed a SingleThreadedTaskRunner. This is not the case for iOS which uses a standard sequenced task runner.

This call to fetch promotions happens when showing the rewards panel but the rewards engine may not be initialized until the panel is shown (a sec/privacy requirement to not make API calls to rewards services until a user interacts with the feature), so it was being scheduled instead of executing immediately, thus hitting the threading issue on the brave-core side

Summary of Changes

This pull request fixes #8109

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()
• New or updated UI has been tested across:
  • Light & dark mode
  • Different size classes (iPhone, landscape, iPad)
  • Different dynamic type sizes

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).