Allow android client code to customize preview size.

The bar code scanner was very slow on my 2013 Nexus 7. I suspected in might be image size when I loaded the same app on my Nexus 4 and it was really fast. The Nexus 7 was using a preview size of 1920x1080, and when I turned it down to 320 x something it still recognized bar codes fine but was way faster.

If you think this would be a good addition to the project, but don't care for the technique I used for exposing the option, let me know and I'll adjust it.