fix(cdk/drag-drop): Support for nested drop containers

Fixes a two issues:

- The first fixed issue is that the elements of
cdkDropListConnectedTo previously had to be ordered correctly to support moving items into an
inner drop container. Sepcifically, inner drop containers had to be listed before their outer
ancestors.

- The second fixed issue is that items of an inner container could not be re-ordered
without moving the item out of the container and then back in.

Fixes #16671

Author: aparzi

src/cdk/drag-drop/directives/drop-list.ts

@@ -363,6 +363,10 @@ export class CdkDropList<T = any> implements OnDestroy {
 
   /** Handles events from the underlying DropListRef. */
   private _handleEvents(ref: DropListRef<CdkDropList>) {
+    merge(ref.receivingStarted, ref.receivingStopped)
+      .pipe(takeUntil(this._destroyed))
+      .subscribe(() => this._changeDetectorRef.markForCheck());
+
     ref.beforeStarted.subscribe(() => {
       this._syncItemsWithRef(this.getSortedItems().map(item => item._dragRef));
       this._changeDetectorRef.markForCheck();