Best tool to use to "convert" Android apk into runnable web app(let)?

I recently wrote a very simple apk app for a client - that runs nicely on their Android devices. They have since asked for it to be a runnable in web browser app (preferably phone and desktop). What would the best idea for this be?

As it's already in Java- perhaps a web applet? Are there any considerations here? Or is there anything else that I can use? I've also heard to stay away from emulating the app within a web browser as it can be slow...

Sorry for asking a "unorthodox" question, but I know that SO people normally have a few suggestions that can put me on the right path...