Increase test coverage for Vault.js

### What is this about?

The file `app/core/Vault.js` has poor test coverage. The KeyringController v16 update required changes that further decreased coverage. However, adding more tests was blocked by challenges encountered using KeyringController in a test file.

As of https://github.com/MetaMask/core/pull/4362, we should have an easier time using the KeyringController in unit tests. Once that PR has been merged and released, we should update the KeyringController again in mobile, then follow up with additional unit tests for `app/core/Vault.js` that increase coverage to 100% for that file.

### Scenario

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### Design

_No response_

### Technical Details

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### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [x] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

Relates to https://github.com/MetaMask/metamask-mobile/pull/9570