Soak the rice in plenty of water of 2-4 hours. Drain, discarding the water. Combine the rice, hotwater, coconut milk, sugar, and grated coconut in a saucepan. Cook over low low heat for 20 to 25 minutes, stirring frequently, Remove from the heat and let cool slightly. Refrigerate for one hour. Mold rice in contor of four dessert plates, and place the mango slices on the side.