search-provider: Concatenate search terms with AND

Currently the provider will match all panels that match at least
one search term. This differs from the search pattern we use in
GNOME Shell and other GNOME applications, where a result has to
match all search terms. The latter allows users to narrow down
search results by adding additional terms, which is generally
more useful than potentially adding additional results.

https://bugzilla.gnome.org/show_bug.cgi?id=694960