The `Backup` class should take messengers instead of controller references

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### What is this about?

We want to reduce coupling between different pieces of the platform, and the messaging system is our tool to do this. Currently, the `Backup` class receives direct references to controllers. While this is not egregious, it does misalign with how we make use of controllers in other places, such as other controllers in general. We should have it take controller _messengers_ rather than the instances themselves.

When the [messenger delegation ADR](https://github.com/MetaMask/decisions/pull/53) is complete, we might also consider turning `Backup` into a service class and having it take a single messenger, and then delegating `:getState` actions from all of the input controllers to that messenger.

### Scenario

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### Design

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### 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

- [ ] 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
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### References

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Start by locating the Backup class and the controller references it currently receives, then read the messenger delegation ADR linked in the issue. Trace how the relevant controller messengers expose the needed actions; done means Backup accepts messengers instead of controller instances while preserving its existing behavior.

Written by the indexing model from the issue text.