bug(MatTooltip): A11y - Screen readers landmark issue with matTooltip

#### Reproduction
When using the `matTooltip` screen readers confused about the landmark and duplicate the announcement (e.g. main, list, group etc.).
This can make a lot of confusion for visually impaired users.

https://stackblitz.com/edit/angular-gizwqa?file=src%2Fapp%2Ftooltip-overview-example.html

Steps to reproduce (in the example above):
1. Navigate to the first button with keyboard (using Tab key).
2. Screen reader will announce the new landmark, the button and the tooltip: _My buttons grouping Button1 button Info about the action_
3. Navigate to second and last buttons, in the last button the landmark will be announced again.
4. Navigate only throw 2 last buttons - the screen reader will announce only the button as expected.
5. Navigate to the first button - the landmark will be announced again (and so on the last 3 button).


#### Expected Behavior

Screen readers will announce the landmark only once (when entering to the landmark)
In the example:
- My buttons grouping Button1 button Info about the action
- Button2 button
- Button3 button

#### Actual Behavior

Screen readers duplicated the announcement.
In the example:
- My buttons grouping Button1 button Info about the action
- Button2 button
- My buttons grouping Button3 button

#### Environment

- Angular: 11.0.2 (also in V10)
- CDK/Material: 11.0.1
- Browser(s): Chrome V87
(tested with NVDA in Firefox and worked as expected).
- Screen readers: NVDA V2020.2, JAWS 2020

Öffne den verlinkten StackBlitz und beginne mit src/app/tooltip-overview-example.html; reproduziere die Ansagen mit den aufgeführten Screenreadern und den Tastaturschritten. Verfolge das beteiligte matTooltip-Verhalten bei der Landmark-Ansage und überprüfe anschließend, dass das Beispiel die Landmark nur einmal ansagt und der erwarteten Ausgabe entspricht.

Vom Indexierungsmodell aus dem Issue-Text verfasst.