[CommCoreModule] avoid calling Backup Service to get backup secrets

Summary:
Part of ENG-9656

It was identified in https://phab.comm.dev/D13849#387643.

We don't need to call the Backup Service, we have the data locally.

retrieveBackupKeys will be refactored later in the stack and used in the Restore protocol to fetch the User Keys from the backup service.

Depends on D14001

Test Plan: Tested creating backup and secondary device login

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14002