[Snackbar] Not visible if keyboard is displayed (touch devices)

#### Bug:
Snackbar is hidden when the virtual keyboard is opened.
Tested on iOS 11

#### What is the expected behavior?
Snackbar is placed on the bottom of visible screen, above keyboard

#### What is the current behavior?
Snackbar is placed on behind keyboard, not visible

#### What are the steps to reproduce?
Stackblitz Editor: https://stackblitz.com/edit/angular-material2-issue-brodsj

0. Open https://angular-material2-issue-brodsj.stackblitz.io on a touchscreen device with virtual
keyboard.

Case 1:
1. Tap "Open snackbar" link
2. Keyboard is displayed hidding the snackbar

Case 2:
1. Tap "Open same snackbar" button -> The snackbar is displayed and visible
2. Tap "The placeholder" input field -> Keyboard is displayed hidding the snackbar

#### What is the use-case or motivation for changing an existing behavior?
We want to notify the user from some action AND request immediate input.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular 5+

#### Is there anything else we should know?
Tested on iOS11, Safari

从链接的 StackBlitz Editor 复现开始，在打开虚拟键盘的 iOS 11 Safari 触摸屏上验证两种 Snackbar 情况。跟踪复现中的 Angular Material Snackbar 实现，然后确认 Snackbar 仍显示在可见屏幕底部、键盘上方。

由索引模型根据 Issue 内容生成。