Overlays behave erratically when using Virtual Scrolling

#### What is the expected behavior?
Users should be able to utilize Virtual Scrolling with the Tree without worrying about other functionality breaking.

#### What is the current behavior?
When using Tooltips on Tree Nodes in combination with Virtual Scrolling, the Tooltips appear improperly without context.

#### What are the steps to reproduce?
Apply Virtual Scrolling to a Tree and then expose Tooltips on the Tree Node Definitions.

[Demo](https://stackblitz.com/edit/simplified-material-tree-with-virtual-scroll-xkvghx?file=app%2Ftree-flat-overview-example.html) - Mousing over any of the items in the Flat Tree should display a Tool Tip containing the Node's name, seemingly randomly it does accurately display the name in the proper place, most of the time you're left with a small gray box.

Edit: I have updated the demo to include a behavior where clicking on the text of a node will display a Snack Bar containing the node's name. This is also appearing as a gray bar at the top of the screen in many situations. This seems to be related to overlays as a whole in this context.

まず、リンクされている StackBlitz デモを実行し、仮想スクロールを使用する Angular Material Tree で tooltip と snack-bar の失敗を再現します。項目が再利用される間に Tree ノードの overlay がどのように動作するかを追跡します。tooltip に正しいノード名が正しい位置に表示され、snack bar が通常どおり表示されれば完了です。

索引モデルが issue の本文から書いたものです。