Add messageBox to proguard-rules.pro (#10570)

I missed this out from the previous update, oops!

android-project/app/proguard-rules.pro

@@ -33,6 +33,7 @@
     boolean isScreenKeyboardShown();
     boolean isTablet();
     void manualBackButton();
+    int messageboxShowMessageBox(int, java.lang.String, java.lang.String, int[], int[], java.lang.String[], int[]);
     void minimizeWindow();
     int openURL(java.lang.String);
     void requestPermission(java.lang.String, int);