[identity] Augment get_keyserver_keys to use devices table

Summary:
Similiar to D10303 - updated the function to try getting keyserver keys from devices table first, then fall back to the users table.

This code can be greatly simplified once we remove "legacy" devices attribute from users table.

Depends on D10303

Test Plan: Repeated steps from D10303, just made sure the device is of type keyserver. Also added one more non-keyserver device to make sure filtering is working.

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10304