Clear hashes when leaving app

[alexmlong]

We don't want it to be possible for someone to restart the app after it was used and still see all of the hashes being displayed and being copyable to clipboard. So, either just find a way to close the whole app, or at least clear the secret and hashes when the user leaves the app.

[alexmlong]

It looks like all I need to do is call finish() in onStop(). However, erring on the side of information security, I'd say definitely test this to see if there are ways to get out of the app without triggering onStop(), and also, clear the text fields and hash listview in onStop() as well, just in case.