Figure out how to stop more than 1 tone's playback

[dgets]

With commit 7485b5f, playback and stopping playback for a single tone is working without any issues. However, due to the asynchronous nature of things, it is possible to play back more than one tone at a time. When this happens, clicking the associated preset list item to try to stop the tone will only end a single tone. Multiple clicks to the other entries will start playback for more, but you cannot end the playback of other concurrent tones; at least not all of them. This needs to be fixed; unless, due to the way things are set up, it turns into a nightmare, in which case it might be easier to just limit playbacks to one tone at a time until I've gotten through enough Android and Java classes/tutorials to be able to handle this the right way.

[dgets]

I went for limiting playback to one sole tone, for now. Just an interim solution until I learn the proper way to deal with this while allowing multiple tones to be played simultaneously.