ENG-6145.

An alternative to this is to pass username and get BackupID from Rust — in terms of network calls, it should be the same. I implemented this because it required a lot fewer changes because it reused existing code.

Alternatively, we can download only backupID and read backupDataKey and backupLogDataKey (it should exist on primary always) to avoid downloading User Keys - but curious about @bartek's perspective as I don't fully remember why it was implemented that way,

1. Deprecate download_latest_backup_id
2. Update retrieveBackupKeys to have backupID

Depends on D13847

This is the final diff in the stack so testing this end-to-end:

1. iOS and Android (I was using physical devices)
2. username and wallet user
3. testing restore similarly to link

TODO: test Secondary Auth Flow