fix issue 1922 - opens an external app to see the image.

Just launches an intent to open an image.
Weird that there is a duplicate of the class JavaScriptInterface in Reviewer.java and PreviewClass.java.

Also, clicking to play sound in the preview of a card would not work (someone forgot to change the JavaScriptInterface in the PreviewClass to reflect the one in the Reviewer).

Observations:
The javascript I created might seem a bit weird, but if I didn't check for the existence of the parents' onclick I would overwrite the behavior of the playsound button (open the icon in another app but not playing the sound).

I don't know if this approach (calling another app) is good, because zooming now isn't that useful I think. At least it is a solution until we find a better one. What do you think?