refactor: change vault backup option from `WHEN_UNLOCKED` to `WHEN_UNLOCKED_THIS_DEVICE_ONLY`

[NicholasEllul]

Description

This pull request switches the vault backup option from WHEN_UNLOCKED to WHEN_UNLOCKED_THIS_DEVICE_ONLY. This is to match the intended purpose for the functionality. There are have been no indications that the current behaviour has not already been bound to the device.

• WHEN_UNLOCKED: The data in the keychain item can be accessed only while the device is unlocked by the user.
• WHEN_UNLOCKED_THIS_DEVICE_ONLY: Similar to WHEN_UNLOCKED, except the entry is not migrated to a new device when restoring from a backup.

Related issues

Related: https://github.com/MetaMask/accounts-planning/issues/475

Manual testing steps

1. Lock and unlock the wallet

Screenshots/Recordings

Not applicable

Pre-merge author checklist

• [X] I’ve followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• [ ] I've completed the PR template to the best of my ability
• [ ] I’ve included tests if applicable
• [ ] I’ve documented my code using JSDoc format if applicable
• [ ] I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• [ ] 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.