StoredMaterialManager: make getMaterial non-const

First step in making StoredMaterialManager const-correct ("thread-safe") and remove mutables. Make the StoredMaterialManager::getMaterial interface non-const as it modifies the list of materials.

The first commit is the actual interface change. All the remaining commits are just trivial changes in clients (e.g. const StoredMaterialManager* -> StoredMaterialManager*) and were mostly done by search&replace.

The remaining interfaces will be fixed in separate MRs.

This was discussed with @tsulaia and @ssnyder on !54024 (closed).