Look for an item with all required attributes when searching in the Secret Service
If we're sandboxed but the Flatpak is given direct access to the Secret Service, the item doesn't have a xdg:schema
attribute so we look for all items, which means we'll have those of other apps that will be detected as malformed items.
If we always search for items with all the required attributes, we will reduce the number of results.
Technically, this shouldn't be a valid use case, but this would avoid issues like: #1113 (closed).
We could also only accept completely valid items and dismiss others, since having invalid items shouldn't occur, or should not be handled by the app.