ValueAnimator默认的插值器是两边慢，中间快

ValueAnimator源码如下：

// The time interpolator to be used if none is set on the animation
private static final TimeInterpolator sDefaultInterpolator =
new AccelerateDecelerateInterpolator();

@Override
public void setInterpolator(TimeInterpolator value) {
if (value != null) {
mInterpolator = value;
} else {
mInterpolator = new LinearInterpolator();
}
}

如果设置为空，又变为LinearInterPolator线性插值器