Fix an issue where previously set initial state is reset on subsequent calls to AddContinuousVariables.
Fix an issue where previously set initial state is reset on subsequent
calls to AddContinuousVariables.

billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake