Move groupping device by platform details and notif info generation promise to utils in lib/push

Summary: This differential moves a utility function that groups devices by platform details to lib since we will need it for client-sent notifs as well.

Test Plan: check that notifs work on the keyserver.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12476