[cdkDrag] Dragging freezes when mouse over on a disabled button

#### What is the expected behavior?

Element should be smoothly dragged over but dropping of an element should be blocked as it's a disabled button.

#### What is the current behavior?

Dragging freezes over a disabled button.

#### What are the steps to reproduce?

Here is the link of [Slackblitz demo](https://stackblitz.com/edit/angular-kbdjsa).

Start with the linked StackBlitz reproduction and trace the cdkDrag behavior when the pointer passes over a disabled button. Confirm the existing drag-and-drop entry points and tests before changing anything. Done means dragging remains smooth over the button while dropping on it stays blocked.

Written by the indexing model from the issue text.