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>
6 files changed