[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 */
});
}

まず MatMenuTrigger と MatMenu のイベントライフサイクルを読み、特に既存の menuClosed イベントと参照されている _animationDone ストリームを確認します。閉じるアニメーションが完了するタイミングを確認し、両コンポーネントの公開 afterClosed 動作を定義します。タイムアウトや非公開 API に依存せず、メニューが完全に閉じた後にコンシューマーが反応できれば完了です。

索引モデルが issue の本文から書いたものです。