MusicFactory: AssetFileDescriptor closed and method to load music async

This patch add close() method to the file descriptors after the setDataSource call: according to the Android documentation of MediaPlayer.setDataSource:  "It is the caller's responsibility to close the file descriptor. It is safe to do so as soon as this call returns."

This patch also add a new method to load the Music asynchronously like this:
```java
MusicFactory.createMusicFromAssetAsync(aMusicManager, aContext, aFilename, new IMusicLoadedListener() {
	@Override
	public boolean onError(MediaPlayer pPlayer, int pWhat, int pExtra) {
		Debug.e(LOG_TAG, "createMusicFromAssetAsync() KO: MediaPlayer.onErrorListener(...) : " + pWhat + ": " + pExtra);
		return true;
	}

	@Override
	public void onMusicLoaded(final Music pMusic) {
		if (pMusic != null) {
			pMusic.play();
		}
	}
});
```