Summary: [ENG-10126](https://linear.app/comm/issue/ENG-10126/restore-the-old-userdata-key-during-user-keys-restoration).

Alternative is to do it at the end of `getBackupUserKeys` call, however I decided to do it like it because of two reasons:
1. `getBackupUserKeys` is a promise resolved by Rust, we'll need to implement Rust calling `setUserDataKeys` and scheduling on Database thread which is more complex.
2. We might want to call `getBackupUserKeys` just to inspect to return value without setting key and rekeying database, it is better to do it only when needed, directly from restore protcol.

Depends on D14268

Test Plan: Tested later in the stack

Reviewers: bartek, tomek