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

まず、リンク先の StackBlitz で動作を再現し、ダイアログをドラッグした後の MatDialogRef.updatePosition() に注目します。ドラッグと右下の位置がどのように関係するかを追跡し、ドラッグ後に最小化すると、ダイアログがドラッグ前と同じ位置に配置され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。