[identity] Simplify some calls and conversions

Summary:
Part of ENG-5842.
After removing the old device list attributes, there's a lot of code that can be greatly simplified. This diffs does that.

Depends on D10699

Test Plan: Identity integration tests

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10700