Layer visibility defaults to false when portals expect and documentation attests true #2905

Describe the bug
After the updated url-encoding in #2851, Layer visibility defaults to false instead of true. This is a reversal of the old behavior which is still documented in the jsdocs which says visible shall default to true.

Resolution
Because this affects all legacy portals and it's unknown how many use cesium maps (and thus how many would require portal map config updates to remedy this) it's more parsimonious to restore the default to visible behavior that's still documented.