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

Inizia con la riproduzione StackBlitz collegata e riproduci il comportamento relativo all’evento cdkDragStarted su una cdkDropList. Esamina come vengono dimensionati l’elemento trascinato e il segnaposto dell’elenco mentre il contenuto degli altri elementi si comprime. Il lavoro è completato quando il trascinamento con il contenuto degli elementi compresso non causa più problemi al segnaposto e l’elenco torna al suo stato normale dopo il rilascio.

Scritto dal modello di indicizzazione a partire dal testo della issue.