[identity] Reorder device list when updated

Summary:
Resolves ENG-5841. Each time we update the device list (login/register/logout), we should determine primary device and reorder the list accordingly.

Depends on D10400

Test Plan:
Added unit tests for the reordering function. Also tested manually by:

1. Register a user (mobile device)
2. Log in the user on a "web" device
3. Check database (or call GetDeviceListForUser RPC) - last device list entry has the following order (simplified for brevity): ["ios", "web"]

Now for web-only users:

1. Register a user (mobile device)
2. Call logout to remove the device
3. Login with web - a log is displayed: "No valid primary device found..."
4. Log in with mobile again. Check database - last device list entry has the following order: ["ios", "web"]. Mobile was added BEFORE web

Reviewers: varun, michal

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10404