[FR] afterClosed for MatMenuTrigger and MatMenu

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

#### What is the expected behavior?
I want to be able to detect when a menu is completely closed (after the animation is done)

#### What is the current behavior?
there is a `menuClosed` event that is called when menu is about to be clsoed

#### What is the use-case or motivation for changing an existing behavior?
After menu item selected, I want to update a service the affects the displayed menu panel. Current menuClosed is too early, and it will cause the panel to reflect the change during the close animation.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
angular 6.0.6, material 6.3.0

#### Is there anything else we should know?
workarounds:
1. ```js
onMenuClosed() {setTimeout(() => { /* doing stuff */ }, 500);}
1. ```js
@ViewChild(MatMenu) myMenu: MatMenu;

// ...
onMenuClosed() {
this.myMenu._animationDone
.pipe(filter(event => event.toState === 'void'), take(1))
.subscribe(() => {
/* doing stuff */
});
}

Commencez par lire le cycle de vie des événements de MatMenuTrigger et MatMenu, en particulier l’événement menuClosed existant et le stream _animationDone référencé. Confirmez quand l’animation de fermeture se termine et définissez le comportement public de afterClosed pour les deux composants. Le travail est terminé lorsque les consommateurs peuvent réagir une fois le menu complètement fermé sans dépendre d’un timeout ou d’une API privée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.