bug(drag-drop): cdk-overlay-pane is not scrollable

[MursalovAltun]

To reproduce this issue we need to create a material dialog and make it draggable using cdkDrag directive and cdkDragRootElement=".cdk-overlay-pane" applied to the dialog's header.
Therefore in mobile devices we are not able to scroll dialog's content

StackBlitz with the issue - https://stackblitz.com/edit/angular-ivy-nyrwmp

This issue only appears in 9.0.0+ versions.

Expected Behavior

Just drag dialog clicking header

Actual Behavior

Now we are dragging dialog by touching anywhere in the dialog, even by inputs touching

Environment

• Angular: v9+
• CDK/Material: v9+
• Browser(s): Google Chrome Version 83.0.4103.61 (Official Build) (64-bit)
• Operating System (e.g. Windows, macOS, Ubuntu): Android 7.0

[crisbeto]

It seems like 2 different issues are being described here.

1. Based on the issue title, cdk-overlay-pane isn't meant to be scrollable so if you need it to be, you have to override the CSS yourself.
2. Based on the issue description, it seems like you've hit a legitimate bug where cdkDragHandle can't be set on a cdkDrag element. This should be fixed on our end.

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