[identity] Rename verify_initial_device_list to verify_singleton_device_list

Summary: Similarly to the rename in D12586, this makes the function more universal because in fact it verifies singleton list (not necessarily initial).

Test Plan: cargo check

Reviewers: kamil, varun, will

Reviewed By: kamil, varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12587