device: Add support to Identify an user using all the available prints

When a device supports identification we can identify the user for a matching print quite reliably, and this may allow users to login without having to select their username first at greeter.

The operation can be performed at any point, even when a device is already claimed, but of course it must not be used by another operation. This allows some more agile Identification usage.

In this commit we reuse the verification signals when possible to avoid duplication of the same behavior and making reviewing easier.

This is safe since we ensure that there are not two users with the same prints only when a proper check will be done during enrollment but in the mean time and to be retro-compatible, we'll always double-check that we have no other matching print in our gallery, before returning a succeeded result.

Added various tests covering (hopefully) all the new cases.