code: public void setButtonLoc(PathPoint newLoc) {
          mButtonProxy.setTranslationX(newLoc.mX);
          mButtonProxy.setTranslationY(newLoc.mY);
      }
docs: We need this setter to translate between the information the animator
  produces (a new "PathPoint" describing the current animated location)
  and the information that the button requires (an xy location). The
  setter will be called by the ObjectAnimator given the 'buttonLoc'
  property string.