Use fixed indices for button order

Apps always return 3 strings, even if there is no custom text, so the index should be constant for each button.

The "OK" button is always at index 2.

src/android/app/src/main/java/org/citra/citra_emu/applets/SoftwareKeyboard.java

@@ -147,7 +147,7 @@ public final class SoftwareKeyboard {
                 case ButtonConfig.Single: {
                     final String text = config.button_text == null
                             ? emulationActivity.getString(android.R.string.ok)
-                            : config.button_text[config.button_config];
+                            : config.button_text[2];
                     builder.setPositiveButton(text, null);
                     break;
                 }