[identity] Create function to determine primary device

Summary:
Resolves ENG-5287. Implements a function that determines which device should be primary basing on user's current devices.

Depends on D10399

Test Plan: Unit tests covering all cases from the ENG-5844 testing task.

Reviewers: michal, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10400