[native] Notify primary device on registration finish

Summary:
After successful registration (D11295), secondary device notifies primary device that it has performed device key upload. At this point, primary device can continue (next diffs)

Depends on D11295

Test Plan: An alert is displayed on primary device after successful device keys upload on secondary device

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11296