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.
Also, if possible, also test calling setMute, getMute, setMediaTime, getVolume, setVolume. For example, even if you can't test if the audio was actually muted, it'd still be worth testing that we don't throw any exception when setMute is called (and that getMute returns the correct value).