[Tabs] Toggling quickly can cause content to be duplicated

#### Bug, feature request, or proposal:

I have had some trouble reproducing this one reliably (it seems to occur more often on my app than in reproductions) but there seems to be a race condition related to the rendering of tab content when it is lazy-loaded.

#### What is the expected behavior?

Tab content should only be displayed once.

#### What is the current behavior?

Toggling tabs very quickly (using arrow keys + space bar) can cause tab content to be drawn multiple times.

#### What are the steps to reproduce?

StackBlitz: https://stackblitz.com/edit/angular-material2-issue-h6pgut?file=app%2Fapp.component.ts

I am intentionally trying to be awkward in terms of timing (ie. setting the `selectedTabIndex` in a timeout), but this behaviour still should not happen.

I have recorded a reproduction in this video: https://gfycat.com/ReadyFairBasil
Note that at the start there is only one instance of each tab's content but by the end the second tab's content is duplicated.

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

Bug

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

Latest StackBlitz (Angular 6-rc2, Material 6-rc1)

#### Is there anything else we should know?

I could only reproduce by using lazy-loaded tabs + NgForTrackBy + OnPush + window.setTimeout, but these may not be necessary. I also had to use keyboard selection to toggle fast enough to trigger the issue.

Beginne damit, die StackBlitz-Reproduktion in app/app.component.ts auszuführen und eine schnelle Tastaturauswahl mit verzögert geladenen Tabs, NgForTrackBy, OnPush und setTimeout durchzuführen. Verfolge den Rendering-Pfad der Tab-Inhalte, um zu ermitteln, warum Inhalte mehr als einmal erscheinen; abgeschlossen ist die Aufgabe, wenn die Reproduktion beim wiederholten schnellen Umschalten genau eine Instanz beibehält.

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