Fix bug when multiple embeds have no or duplicated element id

This bug caused multiple embeds to break when the elements had no id or
duplicated ids. This broke event handling and some interaction features.

Change Service.addOrOverwriteEmbed algorithm that compared elements by
their id so that it now directly compares elements reference equality.

Expand the service init test to include three different tests covering
initialization of multiple embeds with different element id
configurations (with id, without id*, with duplicated id*)

* These two tests will fail without the fix applied to service.ts

Author: oceantume