[identity] Extract device list verification to separate func

Summary:
Some refactor - Extracted verification logic to the caller of apply_device_list_update() to give flexibility of giving different verification criteria for different scenarios (e.g. we want it to be able to replace device list with a singleton during backup restore, but not when primary device is adding a new device)

Depends on D11685

Test Plan:

• Manually called UpdateDeviceList RPC. Behaves the same before and after this diff
  • Valid device list (1 device added)
  • Invalid update (2 devices added)
  • Invalid update (no changes)
  • Invalid update (device removed)
• Integration test still passes

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11686