Add IDroid Interface

cis237assignment3/IDroid.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace cis237assignment3
+{
+    interface IDroid
+    {
+        void CalculateTotalCost();
+
+        decimal TotalCost { get; set; }
+    }
+}