Multichain: Make getCurrentChainId Selector Contextual

### What is this about?

At present the `getCurrentChainId` function within the extension always returns the globally selected chainId from the `NetworkPicker`. We need to update this function to return the chainId of the dapp/page context that lives on the `state.metamask.domains` object. In the case that the user is not connected to a dapp, we can return the globally selected chainId, which is also still required for legacy dapps.

### 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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