[Tooltip] Tooltip does not vanish after route navigation with reusable routes

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

Bug

#### What is the expected behavior?

A displayed tooltip should vanish, after navigating to a route with a different component.

#### What is the current behavior?

When a RouteReuseStrategy is implemented, then the tooltip doesn't vanish if the original route is detached while the tooltip was visible.

#### What are the steps to reproduce?

StackBlitz example: https://stackblitz.com/edit/angular-yvcjzp

A tooltip is displayed on button "Goto Page 2". Clicking on the button will navigate to page 2. This will causes the tooltip from page 1 to be stuck on the screen. It will only vanish once the user navigates back to page 1 again.

Disabling reusable routes by removing the custom RouteReuseStrategy will lead to the the expected behavior again:

```
providers: [
// remove this, and the tooltip starts to work properly
{ provide: RouteReuseStrategy, useClass: CustomReuseStrategy }
],
```

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

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

"dependencies": {
"@angular/animations": "6.0.0",
"@angular/cdk": "^6.0.0",
"@angular/common": "6.0.0",
"@angular/compiler": "6.0.0",
"@angular/core": "6.0.0",
"@angular/forms": "6.0.0",
"@angular/http": "6.0.0",
"@angular/material": "^6.0.0",
"@angular/platform-browser": "6.0.0",
"@angular/platform-browser-dynamic": "6.0.0",
"@angular/router": "6.0.0",
"core-js": "^2.4.1",
"rxjs": "^6.1.0",
"rxjs-compat": "^6.1.0",
"zone.js": "^0.8.26"
},

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

Start with the linked StackBlitz reproduction and inspect the tooltip behavior during navigation with the custom RouteReuseStrategy enabled. Compare it with navigation after removing that provider; done means a visible tooltip from page 1 disappears when navigating to page 2, without requiring a return to page 1.

Written by the indexing model from the issue text.