Sidenav dynamic mode update backdrop

[patrickhousley]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When the mode input is dynamically updated from over to side, the backdrop should be removed if it is currently present on the screen.

What is the current behavior?

When the mode input is dynamically updated from over to side, the backdrop is not removed if it is currently present on the screen.

What are the steps to reproduce?

Providing a Plunker (or similar) is the best way to get the team to see your issue.
Plunker template: https://goo.gl/DlHd6U

What is the use-case or motivation for changing an existing behavior?

This should be considered a bug fix.

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

Angular 4.3.6
Material 2.0.0-beta.10
OS X 10.11.6
TS 2.3.4
Chrome 60.0.3112.113

Is there anything else we should know?

[willshowell]

See #6271

[josephperrott]

Closing as duplicate of #6271

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