bug(mat-dialog): element with fixed position jumps when dialog opens.

#### Reproduction

Use StackBlitz to reproduce your issue:
- Components: https://stackblitz.com/edit/angular-eyckbt?file=src/app/dialog-overview-example-dialog.html

Steps to reproduce:
1. Just open dialog which contains element with fixed position inside it.


#### Expected Behavior
Element is positioned correctly when dialog opens.

#### Actual Behavior
Element jumps from one position to correct one.

#### Environment

- Angular: in stackblitz link
- CDK/Material: in stackblitz link
- Browser(s): Chrome v94
- Operating System (e.g. Windows, macOS, Ubuntu): Windows 10 latest

Start with the linked StackBlitz reproduction and inspect src/app/dialog-overview-example-dialog.html, then reproduce the jump in Chrome v94 when opening the dialog. Trace the dialog's positioning behavior for the fixed-position element; done means the element is correctly positioned immediately when the dialog opens without regressing other dialog behavior.

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