[identity] Verify new flow migration device list update

Summary:
Address ENG-9694

• For old flows: Should allow reordering primary device under the following conditions:
  • Previous device list was unsigned.
  • Reorder only makes the calling device primary (only one swap, no more changes).
• Otherwise, if previous device list was signed, it should act as before

Depends on D14087

Test Plan:

• Commtest
• Manual testing during development of device list migration handler

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14088