Android: Implement open and save file dialog.

android-project/app/proguard-rules.pro

@@ -49,6 +49,8 @@
     int showToast(java.lang.String, int, int, int, int);
     native java.lang.String nativeGetHint(java.lang.String);
     int openFileDescriptor(java.lang.String, java.lang.String);
+    boolean showFileDialog(java.lang.String[], boolean, boolean, int);
+    native void onNativeFileDialog(int, java.lang.String[], int);
 }
 
 -keep,includedescriptorclasses,allowoptimization class org.libsdl.app.HIDDeviceManager {