CDKDragAndDrop: Collapse cdkDropList item contents on cdkDragStarted event

### (Marked as troubleshooting since I wasn't sure if there was a known solution for this case)
#### What are you trying to do?

Collapse/hide the contents of all items in a cdkDropList upon starting a drag event, leaving a condensed view of labels that is easier to sort.

#### What troubleshooting steps have you tried?

When starting the drag event, all items collapse except for the item currently being dragged, causing issues in the list placeholder when attempting to drop an item. Once the drop event is complete, the dropped item collapses and the list returns to normal.

#### Reproduction

StackBlitz: https://stackblitz.com/edit/components-issue-bt4kvt?file=app%2Fapp.component.ts

Steps to reproduce:
1. Set up a cdkDropList with items that contain a title and collapsible content panel.
2. Set the cdkDragStarted event for the list items to collapse all of the content panels.
3. Make sure all items in the list are expanded.
4. Begin dragging one of the items.

#### Environment

- Angular: 8.2.0
- CDK/Material: 8.2.3
- Browser(s): Chrome
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

Comienza con la reproducción enlazada de StackBlitz y reproduce el comportamiento relacionado con el evento cdkDragStarted en una cdkDropList. Inspecciona cómo se dimensionan el elemento arrastrado y el marcador de posición de la lista mientras el contenido de los demás elementos se contrae. Se considera terminado cuando arrastrar con el contenido de los elementos contraído ya no provoca problemas con el marcador de posición y la lista vuelve a su estado normal después de soltarlo.

Escrito por el modelo de indexación a partir del texto del issue.