All cdkDrag items in a CdkDrop element are assumed to be members of a reorderable list

Currently all `cdkDrag` items in a `CdkDrop` element are treated as reorderable items in a list. While the underlying data structure is still left to the developer, the `CdkDragDrop` system updates the UI temporarily with changes to the "list order" even if the items are not supposed to be treated as a list.

Instead, we should have two directives for `CdkDrop` areas, `cdkDropZone` and `cdkDropList`. The former, does not do any automatic UI changes during the drag action and the latter does the current action of reordering items in the list.

Start by tracing the CdkDrop, CdkDragDrop, cdkDropZone, and cdkDropList entry points described in the issue. Determine how the current drop behavior updates list order, then define the separate behaviors so cdkDropZone leaves UI order unchanged while cdkDropList retains reordering.

Written by the indexing model from the issue text.