Since the introduction of the new Bitcoin accounts in the
account-list-menu, we introduced a regression regarding the "Show
conversion in testnets" flag (for both Bitcoin and Ethereum accounts).

This PR fixes that.

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Fixes:
- MetaMask/accounts-planning#545

1. `yarn start:flask`
2. Enable Bitcoin support (Settings > Experimental > Enable "Add a new
Bitcoin*" flags
3. Creates mainnet + testnet Bitcoin accounts
4. Check that the "Show conversion in tesnets" flag is off
5. Open the account menu list (click the header top bar, on the
currently selected account)
6. You should not see any USD prices except for mainnet accounts (you
must be on a mainnet EVM networks to see that for your EVM account)
7. Enables "Show conversion in tesnets" flag
8. Repeat step 5
9. You should now see USD prices also for testnet accounts/test assets

![SCR-20240730-mjje](https://github.com/user-attachments/assets/17b44944-44c8-4480-ae37-5c1236b17a81)

![SCR-20240730-mjme](https://github.com/user-attachments/assets/4cadb111-064c-46d8-b961-5623e2f509a6)

![SCR-20240730-mnlq](https://github.com/user-attachments/assets/32ca75a5-0525-40be-90fe-d05fd83033c3)

- [x] I've followed [MetaMask Contributor
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Standards](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
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if applicable
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guidelines](https://github.com/MetaMask/metamask-extension/blob/develop/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

---------

Co-authored-by: Daniel Rocha <daniel.rocha@consensys.net>