bug(cdk/drag-drop): Cannot drag without scrolling on mobile devices.

[shervin01]

This change (#21227) has broken dragging on mobile devices. On mobile dragging will also trigger scroll. To reproduce go here on a mobile device or use chrome's toggle device tool bar: https://material.angular.io/cdk/drag-drop/examples

Reproduction

Steps to reproduce:

1. go here on a mobile device or use chrome's toggle device tool bar: https://material.angular.io/cdk/drag-drop/examples
2. then scroll down a bit.
3. then try to drag one of the boxes. You can see that screen also scrolls.

Expected Behavior

Screen should not scroll while dragging. You can see the desired behavior here: https://v10.material.angular.io/cdk/drag-drop/examples

Actual Behavior

Screen/Document scrolls while dragging

Environment

• Angular: 11.1.1
• CDK/Material: 11.1.1
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): Tested on Android as well as Desktop chrome with toggle device set to iPhoneX

[crisbeto]

It's unlikely that #21227 introduced it since stopPropagation doesn't have an effect over whether scroll events are blocked. This likely happened in #21382 which accidentally made it so scrolling only gets blocked on the second touchmove event. I've submitted #21752 which should resolve it.

[shervin01]

@crisbeto Yes you are correct. Thanks for looking into this.