[AI] Spatially loaded objects in a WP map that are unloaded do not contribute to the navmesh when Build Paths is run

Using Build Paths does not create a navmesh that also includes any Actors or Objects that are unloaded and spatially loaded. The navigation system appears to load the world within the navigable bounds and the actors will be loaded, but the navmesh build does not include them.

Increasing the verbosity of LogNavOctree to Verbose shows that the spatially loaded actors are not registered with the octree until after the navigation building completes. This has been tested and happens with projects from 5.4, 5.5, and 5.6 as well as Main.

1. Open a WP level or create a new one
2. Add plane to the level
   1. Uncheck Is Spatially Loaded on plane
3. Add Cube actor to level
   1. Make sure Is Spatially Loaded is checked for cube
   2. Place the cube on the plane
4. Add Navmesh Bounds to level and scale to encompass all of the plane
5. In Editor Preferences->Level Editor->Miscellaneous, uncheck Update Navigation Automatically
6. Unload the region containing the plane and cube from the level
7. Build Paths

Expected Result: The navmesh is generated on the plane and is cut out around the cube.

Actual Result: The navmesh is generated on the plane and does not account for the cube.