[Tooltip] Change detection runs after showing tooltip

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

Bug

#### What is the expected behavior?

Hovering an element with a `matTooltip` should not trigger change detection on this element if it is an `OnPush` element and its `@Input()`s does not change.

#### What is the current behavior?

Hovering an element with a `matTooltip` triggers change detection on this element.

This only happens if `matTooltip` is defined using `[matTooltip]="..."` and not when it is defined using `matTooltip="..."`

#### What are the steps to reproduce?

See https://stackblitz.com/edit/angular-material2-issue-ocg9sj
- pass the mouse on the red `div`
- you get logs everytime the function needed to draw the component are called
- you don't get any log from `ngOnChanges`
- I would expect that since the `@Input()` didn't change, the component shouldn't be redraw!

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

Prevent useless redraws

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

cf stackblitz

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

Beginne mit der verlinkten StackBlitz-Reproduktion und untersuche das Verhalten von matTooltip, wenn sein Wert mit [matTooltip] an ein OnPush-Element gebunden ist. Vergleiche es mit dem statischen Fall matTooltip="..." und beobachte die Change-Detection-Logs beim Hover. Erledigt bedeutet, dass Hovering keine unnötige Change Detection auslöst, wenn sich die @Input()-Werte nicht geändert haben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.