Use audio element to play audio instead of the Audio API.

Audio API doesn't support AMR file on Firefox OS device, so I changed the implementation to use audio tag.
The new implementation fixes bug #758 to make `AudioPlayer::getMediaTime` returns the duration when the audio ends.

It also makes improvement on TestAudioPlayer to avoid intermittent test failure.
We have another file to test volume control functions: https://github.com/andreasgal/j2me.js/blob/f07e06a58b20202a5017fea934956f6dee3cfc41/tests/javax/microedition/media/control/TestVolumeControl.java

:-)