Fix SceneImportSettings perf issues

[hcoura]

Scene Importer was generating collider mesh at every tick when Physics options was enabled, this makes it only do it when changing settings related to INTERNAL_IMPORT_CATEGORY_MESH_3D_NODE.

Honestly I am not 100% confident the actual implementation is how something like this would be done in the project but I couldn't find some clear pattern to replicate.

This closes #68301

[clayjohn]

I've left a suggestion that I think helps the codestyle/clarity. In terms of content, this change makes sense to me. We definitely do not want to be creating a collider every tick

[clayjohn]

Looks fine to me, but I am not expert in the importer, so it would be nice to have a review from someone else more knowledgeable as well.

[akien-mga]

Thanks!