[lib] refactor useClientBackup to not use user identifier from the store

Summary:
ENG-8195.

Previous API was wrong, we don't have username in the store when performing restore

Depends on D14079

Test Plan: Play with backup menu and see if this still works

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14080