[native] make sure start_backup_handler is called only once in BackupHandler
Summary:
ENG-10001.
The asynchronous nature of this effect caused this promise to be spawned a couple of times, resulting in the starting handler multiple times.
Depends on D14095
Test Plan:
Login -> logout -> login = no issues about backup upload, user is not logged out automatically.
Added some logs to make sure there is only one call to startBackupHandler
Reviewers: bartek, tomek
Reviewed By: bartek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D14096