Check if device already claimed Client can claim only unclaimed device. Cloud can claim any device. Claim with kNone is not allowed. BUG=25766815, 26143922 Change-Id: Id92168b7f7c290509e672a659f09b7d06af37b76 Reviewed-on: https://weave-review.googlesource.com/1960 Reviewed-by: Vitaly Buka <vitalybuka@google.com>