Only include attachments with editable feature layers when taking offline.

For offline maps, a feature layer is considered to be editable if the feature service has capabilities that include any of create, update or delete. A read-only layer is one that supports sync, but does not have any of create, update or delete capability. Service capabilities are accessible from service infos see ArcGISFeatureServiceInfo.featureServiceCapabilities or ArcGISFeatureLayerInfo.capabilities.