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.
Could you add a test like this after resume and after replay? Or is it too unreliable?