How to convert LayerDrawable to Bitmap?

Just draw it on a Canvas which is backed by a Bitmap.

int width = drawable.getIntrinsicWidth();
int height = drawable.getIntrinsicHeight();
Bitmap bitmap = Bitmap.createBitmap(width, height, Config.ARGB_8888);
Canvas canvas = new Canvas(bitmap); 
drawable.setBounds(0, 0, canvas.getWidth(), canvas.getHeight());
drawable.draw(canvas);