Improving the example of Android java wrapper

[DovgopolsSerj]

The Java MDKPlayer class in the example is very small.
I am not familiar with C and it is difficult to figure out how to complement the API.
Simple methods like setMute and setVolume turned out to be quite simple to add.
But it is not clear how to implement the onEvent callback.
Can you update the example?
Or just give an example of transferring a callback to native code.