feat(material): bottom-app-bar component

## Feature Description
https://material.io/design/components/app-bars-bottom.html

Mentioned in https://github.com/angular/components/issues/13964#issuecomment-441044989

Mentioned in https://github.com/angular/components/issues/408#issuecomment-433441313

### Other Information
MDC does not yet have a `mdc-bottom-app-bar`, but there is a [feature request](https://github.com/material-components/material-components-web/issues/2659) for one.

There is a separate feature request for a `mat-top-app-bar`: https://github.com/angular/components/issues/13964

This has been opened as people keep conflating Bottom App Bars with Bottom Navigation (which we are tracking separately in https://github.com/angular/components/issues/408). Now we can point them to this issue.

首先阅读链接的 Material Design bottom app bar 规范以及所引用的 MDC 功能请求。将请求的组件与单独的 top app bar 和 bottom navigation issue 进行比较。该 issue 没有指定文件、测试或实现入口；除添加该组件外，没有定义完成标准。

由索引模型根据 Issue 内容生成。