Put new items into proper childEntities collection when using InvisibleCollectionFetch

When using ETM with InvisibleCollectionFetch collection fetch strategy,
new added items from resource syncing ends up in wrong collection
and just empty data are displayed in the view.

Debugging this I discovered that the newly created items must be stored
with "m_rootCollection.id()" as key instead of "collection.id()"
(m_childEntities[collection.id()] is actually empty/non-existant
even though the model has valid items from this collection)
as the model is actually just a flat list.

REVIEW: 107153