Update gradle build system to a working state.

Hi,

this is my first pull request so I don't know if I'm doing it right ;-)

I've updated the gradle build system for pd-for-android so that it is actually working. Currently you can build PdCore, CircleOfFifths, ScenePlayer and Voice-O-Rama. I don't know if there's something missing.

I'll send a corresponding pull request for btmidi.

Regards,

Felix Homann