[native] unify username and wallet backup restore

Summary:
ENG-6145.

This allows us to decrease code duplication (next diff) and use the same interface for wallet/username users.

Additionally, moving restore methods to BackupMenu as this is for now only for testing purposes and we'll need something different for actual backup.

Depends on D13840

Test Plan: Tested in D13849

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13844