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 file an issue to add the tests once https://bugzilla.mozilla.org/show_bug.cgi?id=816041 is resolved?