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?

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Inizia con la riproduzione StackBlitz collegata e analizza come le finestre di dialogo aperte all’interno di mat-sidenav-container applichino le strategie di scorrimento block e noop, con e senza backdrop. Conferma la correzione riproducendo le combinazioni segnalate e verificando che block impedisca lo scorrimento mentre noop lo consenta.

Scritto dal modello di indicizzazione a partire dal testo della issue.