Fix #7433: Asset auto discovery may show loading indicator when not loading

[StephenHeaps]

Summary of Changes

• Only update isLoadingDiscoverAssets in the Wallet Service observer methods onDiscoverAssetsStarted & onDiscoverAssetsCompleted, otherwise we might set isLoadingDiscoverAssets to true but rate limiting in core prevents asset auto-discovery from running (because we just discovered assets moments prior).

This pull request fixes #7433

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

Test Plan:

1. Unlock your wallet
2. Wait for loading spinner beside Assets header to disappear
3. Lock your wallet & immediately unlock it again
4. Verify loading spinner is not displayed, or is displayed and removed as expected

Screenshots:

Simulator.Screen.Recording.-.iPhone.14.Pro.-.2023-05-10.at.17.20.03.mp4

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).