BugFix: Make Round Serializable to prevent application crash on switching to another app.

app/src/main/java/me/zobrist/tichucounter/Round.kt

@@ -1,6 +1,8 @@
 package me.zobrist.tichucounter
 
-class Round() {
+import java.io.Serializable
+
+class Round(): Serializable {
     var scoreA: Int = 0
     var scoreB: Int = 0