code: private void animatePropertyBy(int constantName, float byValue) {
          float fromValue = getValue(constantName);
          animatePropertyBy(constantName, fromValue, byValue);
      }
docs: Utility function, called by the various xBy(), yBy(), etc. methods. This method is
  just like animateProperty(), except the value is an offset from the property's
  current value, instead of an absolute "to" value.
  
  @param constantName The specifier for the property being animated
  @param byValue The amount by which the property will change