Implement audio capture API

Please visit the following url to run the test MIDlet:
`http://localhost:8000/index.html?jars=tests/tests.jar&midletClassName=org.mozilla.AudioRecorder`

you can set the pref `media.navigator.permission.disabled` to `true` to disable the permission prompt.
In this and other functions, I'd move the code that uses AudioRecorder to the AudioRecorder class itself, for example in this case to AudioRecorder::pause
We could do this in a followup refactoring though