SearchApiViewsQuery should implement get_result_entities()

So, in #1172970: provide a unified way to retrieve result entities fago and you gave Views get_result_entities(), as a great way to get all entities associated with the query.
And that's exactly what we need in both Entityreference and Entityreference View Widget.
However, SearchApiViewsQuery doesn't implement it (yet), and for that single reason a search api view can't be used to select entities