Fix Views fulltext filter operators

I've only now noticed (since I never use it, like probably most other people) that when setting "Use as" to "Search filter", the "Search: Fulltext search" filter issues wrong conditions. Specifically, the selected Views operator (AND, OR or NOT) is used as the Search API query operator (which should be "=" or "<>").
Since service classes seem to default to "=" without complaints, this doesn't seem to cause any real problems, but we should still fix it.