erp5_knowledge_pad: Fix inconsistency with preferred_height and preferred_width in google_search gadget

ERP5Type.ConsistencyMessage for PropertyTypeValidity type_check on portal_gadgets/google_search (message: Attribute preferred_width should be of type string but is of type <type 'int'>)
ERP5Type.ConsistencyMessage for PropertyTypeValidity type_check on portal_gadgets/google_search (message: Attribute preferred_height should be of type string but is of type <type 'int'>)