bug(MatDialogRef): updatePosition is not consistent for previously dragged dialog

#### Reproduction

The `updatePosition()` doesn't seem to be consistent if the dialog is previously dragged.

It is my understanding, according to the documentation that `updatePosition({bottom: '5px', right: '0px'})` will place my dialog at the bottom right of its container (`cdk-overlay-container` if I'm right ?).

[Here is a forked StackBlitz from the documentation](https://stackblitz.com/edit/angular-hc5lgp?file=app%2Fdialog-overview-example.ts) with, of course, minor changes.

With it, please:
1. open the dialog
2. click on the "minimize" button next to the title (dialog goes to the bottom right of the screen as expected : 👍 nice).
3. click on the "maximize" button next to the title to get the dialog back to it initial place. Also 👍 nice.

so far so good !

4. drag the dialog downwards (or in any direction you want) generously and minimize it again.


#### Expected Behavior

The dialog should be at the exact same place as it was after step 2.

#### Actual Behavior

The dialog's position has been updated, but with an additionnal translation.
(That additionnal translation seems to be of the same length as the previous drag.)

Worst : you can get the dialog outside of the screen if your drag was too generous, preventing you from dragging it back on visual.

#### Environment

- Angular: 9.1.7 (but discovered and reproduced in 7.3.4)
- CDK/Material: 9.2.4 (but discovered and reproduced in 6.4.1)
- Chrome 83.0.4103.61 64bits
- Windows 10 64bits

Inizia riproducendo il comportamento nello StackBlitz collegato, concentrandoti su MatDialogRef.updatePosition() dopo aver trascinato la finestra di dialogo. Traccia l’interazione tra il trascinamento e la posizione in basso a destra, quindi verifica che la minimizzazione dopo un trascinamento posizioni la finestra di dialogo nella stessa posizione di prima del trascinamento.

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