[native][lib] merge BackupDataKeyMessage with DeviceListUpdateSuccess
Summary:
ENG-9905.
This diff merges types and moves retrieving keys to a different step.
This change is not backward compatible but this Flow hasn't been launched yet so we not breaking anything.
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 D13964
Test Plan: Tested later in the stack - D13968
Reviewers: bartek, tomek, varun
Reviewed By: bartek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D13965