Return length of opponents' concealed kongs

[katsoohoo]

On the server side, concealed kongs from other players are currently structured below.

  "players": {
    "0": {
      "hand": 8,
      "concealed": [
        "kong",
        "kong"
      ],

We should include the length of the kong to avoid using a magic number on the client.

After fixing the above, the client needs to update OpponentHand to consume the new format.