fix(cdk/drag-drop): avoid retaining disabled handles after they've been destroyed (#21015)
fix(cdk/drag-drop): avoid retaining disabled handles after they've been destroyed (#21015)

The `DragRef` keeps track of which of its handles have been disabled, but if one of
them is destroyed, we still keep its reference until the object is destroyed.

These changes add some extra logic to filter them out.

Fixes #21009.
crisbeto/material2,mmalerba/material2,angular/components,andrewseguin/material2,crisbeto/material2,andrewseguin/material2,mmalerba/material2,mmalerba/material2,andrewseguin/material2,crisbeto/material2,angular/components,andrewseguin/material2,mmalerba/material2,crisbeto/material2,mmalerba/material2,josephperrott/material2,DevVersion/material2,josephperrott/material2,angular/components,angular/components,andrewseguin/material2,DevVersion/material2,DevVersion/material2,josephperrott/material2,josephperrott/material2,josephperrott/material2,DevVersion/material2,DevVersion/material2