bug(CDK - draggable): Dragging element above hidden element

#### Reproduction

https://stackblitz.com/edit/angular-dragging-hidden-elements?file=app%2Fcdk-drag-drop-sorting-example.html

Steps to reproduce:
1. Drag element `3` above `1` - order is wrong because element 2 is hidden


#### Expected Behavior

Hidden elements should be binded to previous visible one and taken into calculations of current index

#### Actual Behavior

Run the reproduction in the linked StackBlitz, starting with app/cdk-drag-drop-sorting-example.html, and confirm the ordering when element 3 is dragged above hidden element 2. Trace the Angular CDK draggable sorting behavior involved in calculating the current index; done means hidden elements remain associated with the previous visible element and the resulting order is correct.

Written by the indexing model from the issue text.