cdkDragStartDelay behavior has changed in version 8.1.0

It seems that the behavior of drag start delay has been changed between Angular CDK versions 8.0.0 and 8.1.0. I think this is a breaking change since it completely changes the way users interact with draggable items. I am not sure if the current behavior is really intended.

#### Reproduction

8.0.0: https://stackblitz.com/edit/angular-cdkdragstartdelay-80
8.1.0: https://stackblitz.com/edit/angular-cdkdragstartdelay-81

Steps to reproduce:
1. Click on any item.
2. Try to drag it as soon as possible.


#### Expected Behavior

In the previous version it did not matter if the mouse was clicked or immediately moved. The dragging started anyway after the specified delay.

#### Actual Behavior

In the current version the item is not being dragged when you move the mouse immediately after the initial click. It only works if you stay at the click position for the specified delay time and then move the mouse.

#### Environment

- Angular: 8.0.0+
- CDK/Material: 8.1.0+
- Browser(s): Google Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Fedora Linux 29

まず、Angular CDK 8.0.0 と 8.1.0 のリンクされた 2 つの StackBlitz 再現を比較し、cdkDragStartDelay と、クリックして直後に移動するシーケンスに焦点を当てます。再現で使用されているドラッグ＆ドロップ実装を追跡し、記載された期待結果および実際の結果に照らして動作を検証します。

索引モデルが issue の本文から書いたものです。