autocomplete does not stick when scrolling

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
The autocomplete drop down list should stick to the bottom of input element when scrolling

#### What is the current behavior?
The drop down list is sticky to position on the screen, it will not follow the input element position when scrolling

#### What are the steps to reproduce?

#### What is the use-case or motivation for changing an existing behavior?

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Windows 10 64 bit
Chrome Version 64.0.3282.167 (Official Build) (64-bit)
Angular 5.1.2
Angular Material 5.0.2
TypeScript 2.7.1

#### Is there anything else we should know?

首先在 Chrome 中使用 Angular 5.1.2 和 Angular Material 5.0.2 重现 autocomplete 的滚动行为，然后追踪 autocomplete 下拉菜单定位的入口点。完成的标准是：下拉菜单始终附着在其输入框底部，并在页面滚动时跟随输入框移动。

由索引模型根据 Issue 内容生成。