[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

Start with the linked StackBlitz Editor reproduction and verify both snackbar cases on an iOS 11 Safari touchscreen with the virtual keyboard open. Trace the Angular Material Snackbar implementation from the reproduction, then confirm the snackbar remains visible at the bottom of the visible screen above the keyboard.

Written by the indexing model from the issue text.