[keyserver] use correct key for device ID

Summary:
resolves: https://linear.app/comm/issue/ENG-4993/use-correct-key-content-ed25519-public-key-as-device-id

Depends on D9220

Test Plan: just tested with flow. it's a pretty straightforward refactor

Reviewers: rohan

Reviewed By: rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9264