MdSelect and MdAutocomplete slow initial render w/ large number of options

For both `MdSelect` and `MdAutocomplete`, the initial render of the component (in the closed state) is very slow even though the panels are closed. We should ensure that any work related to the options is deferred until the panel is opened.

To reproduce this, simply modify the demos to render 500 `md-select` each with 500 options each (there is no problem if there are 500 selects with 1 option each).

首先修改组件演示，以复现 500 个 MdSelect 实例且每个实例包含 500 个选项，然后跟踪 MdSelect 和 MdAutocomplete 的初始关闭状态渲染路径。完成的标准是：与选项相关的工作会推迟到面板打开时执行，同时组件在打开后仍能正确渲染并正常工作，并且大量选项的复现具有良好的响应性。

由索引模型根据 Issue 内容生成。