feat(cdk-drag-drop): Give a visual clue to which droplist can receive a specific dragged item.

[adergham]

Feature Description

Currently,
When dragging an item, the css class cdk-drop-list-receiving is displayed on all the droplists, even those that cannot receive the item due to the enterPredicate output.
This is misleading.

Use Case

Given that a dragged item is restricted to some drop lists, I want to display a visual clue about where I can drop this item

[crisbeto]

Looks it's because isReceiving here only looks at whether any connected siblings are active, but it doesn't account for the predicate. When fixing this, we have to be careful about performance, because isReceiving is invoked inside a data binding.