code: public Animation start(GVRAnimationEngine engine) {
          ((GVRAnimation) getAnimation()).start(engine);
          mIsRunning = true;
          return this;
      }
docs: Start the animation.
  
  Changing properties once the animation is running can have unpredictable
  results.
  
  @param engine
  The global animation engine.
  @return {@code this}, so you can save the instance at the end of a chain
  of calls.