sidenav: scroll strategies not working when mat-sidenav-container is used

#### What is the expected behavior?

When dialog is opened inside mat-sidenav-container scroll strategies should work as usual. Block should block scrolling when dialog is opened, noop should allow scrolling, etc.

#### What is the current behavior?

Scroll strategies don't work when mat-sidenav-container is used. If I open dialog with backdrop and noop scroll strategy, scrolling is still blocked. If I remove backdrop and use block strategy, scrolling is still possible.

#### What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-effkee

Open dialog and try to scroll. Play around with backdrop and scroll settings.

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

Angular, Material and CDK 7.2.2, macos, Chrome (haven't tried other browsers but I assume it's the same)

#### Is there anything else we should know?

/

Beginne mit der verlinkten StackBlitz-Reproduktion und untersuche, wie Dialoge, die innerhalb von mat-sidenav-container geöffnet werden, Block- und Noop-Scroll-Strategien mit und ohne Backdrop anwenden. Bestätige den Fix, indem du die gemeldeten Kombinationen reproduzierst und überprüfst, dass Block das Scrollen verhindert, während Noop es zulässt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.