[lib] refactor notif code to support creating session with multiple devices of one user

Summary:
ENG-9106.

Updating code to avoid calling Identity multiple times.

Depends on D13372

Test Plan: Tested in D13376

Reviewers: bartek, tomek, marcin

Reviewed By: bartek, tomek, marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13373