cdkDropList events not triggered within a mat-table row

#### What is the expected behavior?
When the table row is set to cdkDropList it should treat cdkDrag items within the row as list items, reordering where necessary and triggering cdkDropListDropped on release

#### What is the current behavior?
mat-header-cells with the cdkDrag property inside a cdkDropList mat-header-row are behaving as if they are normal cdkDrag items without a cdkDropList.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-3x3fbt

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
9.1.3

#### Is there anything else we should know?
Similar issue: https://github.com/angular/components/issues/13770

Inizia dalla riproduzione StackBlitz collegata e verifica come cdkDropList e cdkDrag interagiscono all’interno di una riga di intestazione mat-table. Confronta il comportamento con quello dell’issue simile #13770 e verifica se la riga tratta i trascinamenti di mat-header-cell come elementi della lista. Il lavoro è completato quando il riordinamento funziona e cdkDropListDropped viene attivato al rilascio.

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