Finally resolved this... Logic is to get the screen size of the device and calculating the target scaling value based on device orientation. This code has to be added in 'onCreate' method.
// get actual screen size
Display display = ((WindowManager) getSystemService(Context.WINDOW_SERVICE)).getDefaultDisplay();
Point size = new Point();
display.getSize(size);
int width = size.x;
int height = size.y;
// calculate target scale (ORIG_APP_W is max width of image, ORIG_APP_H is max height of image)
double globalScale_Width = Math.floor( ( width / ORIG_APP_W ) * 100 );
double globalScale_Height = Math.floor( ( height / ORIG_APP_H ) * 100 );
// set the scale based on orientation
if(deviceOrientation==getResources().getConfiguration().ORIENTATION_PORTRAIT){
super.appView.setInitialScale( (int)globalScale_Height);
}else if(deviceOrientation==getResources().getConfiguration().ORIENTATION_LANDSCAPE){
super.appView.setInitialScale( (int)globalScale_Width);
}