refreshOnlineBaseLayerIfPresent

Refresh the online Carto CDN raster layer's tile URL when the user switches map style (LIGHT/DARK/GREY). The existing layer captures its tile URL at construction time, so we must remove and re-add to pick the style-specific URL (voyager / dark_all / light_all). Only called when the property is in Carto-online-only mode (no filerId). This is a no-op if no online fallback layer is currently loaded — the guard mOnlineFallbackLayer == null ensures we never re-add Carto for properties that have per-property MBTiles. MUST be called on the UI thread.