Sound.stop() doesn't work

[Mathieu Virbel]

One of our user in python-for-android reported that Sound.stop() wasn't working.
It appear that Channel(i).get_sound() always return None, because get_busy() was returning only True/False, not the serial of the sound.

In addition, multiple call to play() was leading to actual multiple playing. This behavior have been removed to be the same as pygame mixer.

All the changes are available at: https://github.com/kivy/python-for-android/commit/83efd49472c0caf877f1056a0624ec902acf9127

Feel free to merge it :)