close app completely when clicked 'Exit' button (#7655)

* kill app when clicked 'Exit' button

* add super.onDestroy()

* tries to fix temporarily

* Update AndroidLauncher.kt

* add System.exit(0)

* a little fix

core/src/com/unciv/UncivGame.kt

@@ -399,6 +399,8 @@ class UncivGame(parameters: UncivGameParameters) : Game() {
 
         // On desktop this should only be this one and "DestroyJavaVM"
         logRunningThreads()
+
+        System.exit(0)
     }
 
     private fun logRunningThreads() {