[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

Commencez par la reproduction liée de StackBlitz Editor et vérifiez les deux cas de Snackbar sur un écran tactile iOS 11 Safari avec le clavier virtuel ouvert. Suivez l’implémentation de Angular Material Snackbar à partir de la reproduction, puis confirmez que la Snackbar reste visible en bas de l’écran visible, au-dessus du clavier.

Rédigé par le modèle d'indexation à partir du texte de l'issue.