This is the first step of merging BACKUP_DATA_KEY_MESSAGE and DEVICE_LIST_UPDATE_SUCCESS. Initially, I wanted to make this code part of DEVICE_LIST_UPDATE_SUCCESS, after performLogIn is done - that is how this will work in the future. However, we're now running on a limited version of backup and there isn't anything to restore. Going to mention this in ENG-992 in order to revert this.
NOTE: It's not that easy to slice this into diffs, so it's possible that in the middle of the stack protocol is not working, it needs to be reviewed as a whole.
Depends on D13963