fingerprint-dialog: Properly handle claiming errors if we're already claiming

We were a bit too permissive in handling the AlreadyInUse error during
claim, as we assumed it was always us causing it instead of properly
handing the case a device was already claimed by another caller.

So to ensure this is the case we need to avoid multiple calls to claim
until we've finished one.

Unfortunately we can't rely on a cancellable here as we may end up
cancelling the request that succeeded and we'll only get an
AlreadyClaimed error without know if it was us succeeding.

Fixes: #1201