Switch Luxi Query operation to use a proper filter

Until now, since we didn't have a proper type for the encoded query
filters, we were ignoring the filters and handled them as null values
(JSNull and respectively ()). With the current Query2 implementation,
we can actually use correct filters (we can send filters the client
side, and on server side we can decode them).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>