Edit & Remove buttons disappearing when returning from new node preview

Problem/Motivation

Node preview was broken when using a field collection and after coming back from the preview (of a node being added) the widget has lost its values. See #2500993: Entity reference value gets removed when using preview and not saving IEF (and duplicate Field Collection issue #2689499: Previewing node loses field collection entities). But now, if you apply #2500993: Entity reference value gets removed when using preview and not saving IEF, when returning from a new node preview, the "Remove" and "Edit" buttons both are disappearing.

The reason for this misbehaviour:

1. The access to buttons is deferred to FieldCollectionItemAccessControlHandler::checkAccess
2. The access is granted if the host entity allows the access. If the host entity doesn't exist, the access is forbidden.
3. But after previewing a new node the field_collection_item entities are already created but the host is not yet created (we are on /node/add/some-node-type), thus the access is forbidden and the buttons are not displayed.

Proposed resolution

Allow the access to field_collection_item entity when the field knows already the type of its host entity. This means that field_collection_item has already set its host via ::setHostEntity().

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.