Specify namespace for CastServiceChannel

[oleksii-frolov]

Each custom channel is defined by a unique namespace and must start with the prefix urn:x-cast:, for example, urn:x-cast:com.example.custom. For now we don't have an ability to set custom namespace and we use hardcoded namespace.

Source: http://stackoverflow.com/questions/28021536/how-to-specify-namespace-for-sending-message-to-googlecast-receiver-app

[khk624]

FYI, if we add this feature, we also need same thing for javascript bridge.