[native] remove BackupKeys and use QRAuthBackupData

Summary:
Part of ENG-9656

BackupKeys can be misleading and confused with actual Use Keys; additionally, it also contains backupID. This was duplicated with the QRAuthBackupData type, which is the same type, but it's more suitable as it's going to be used only in the context of the Secondary Device Login protocol.

Additionally renaming retrieveBackupKeys -> getQRAuthBackupData

Depends on D14002

Test Plan: Tested creating backup and secondary device login

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14003