Full-screen overlay container doesn't work when fullscreen element is within an overlay

#### Bug, feature request, or proposal:
I believe this might be a bug.
In this example I try to load a snackbar into a container using the `viewContainerRef` config.
https://stackblitz.com/edit/angular-material-tooltip-in-container

#### What is the expected behavior?
The snackbar should align itself to the viewContainerRef that is provided.

#### What is the current behavior?
But the `_createOverlay` function does not look at this config and always creates the overlay using the `GlobalPositionStrategy` instead of `ConnectedPositionStrategy`.

There could be another root cause, and if there is please let me know.

#### What are the steps to reproduce?
I have provided a stackblitz above.

#### What is the use-case or motivation for changing an existing behavior?
There is a hand full of times i would like to load the snackbar in a container.
One in particular is the div has been maximized to full screen (so the window is not visible).
Therefore the snackbar is not visible.

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

リンクされた StackBlitz の再現コードと `_createOverlay` エントリポイントから始めます。snackbar の `viewContainerRef` 設定が、グローバルおよび接続された位置戦略に対してどのように処理されるかを追跡し、その後、フルスクリーンのオーバーレイコンテナ内で snackbar が表示され、位置合わせされていることを確認します。

索引モデルが issue の本文から書いたものです。