Fix for the "Back button can't end Unciv (#1513)" (#1661)

* Callback to exit the game by 'Back' button

* Prompt dialog for the game exit

* Additional strings for translations are generated

* Do not show exit prompt dialog twice

android/src/com/unciv/app/AndroidLauncher.kt

@@ -9,7 +9,9 @@ class AndroidLauncher : AndroidApplication() {
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         val config = AndroidApplicationConfiguration().apply { useImmersiveMode = true }
-        val game = UncivGame(BuildConfig.VERSION_NAME, CrashReportSenderAndroid(this))
+        val game = UncivGame(BuildConfig.VERSION_NAME,
+                            CrashReportSenderAndroid(this))
+                            {this.finish()}
         initialize(game, config)
     }
 }