[identity] Allow UpdateDeviceList RPC to remove devices

Summary:
Updated device list validation logic. Added possibility to remove a device in UpdateDeviceList RPC.

Depends on D11686

Test Plan:
Called the RPC with a few scenarios:

• Add one device - pass
• Add two devices - fails
• Remove one device - pass
• Remove two devices - fail
• Remove primary device - fail
• Both remove and add device - fail
• Keep list unchanged - fail

Reviewers: varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11687