ATOM-16747 RPISystemInterface::GetDefaultScene returns the scene crea… (#5153) (#5389)

* ATOM-16747 RPISystemInterface::GetDefaultScene returns the scene created by PreviewRenderer but not the Main Scene
Deprecate GetDefaultScene() function.
Update all the places which use GetDefaultScene to use Scene::GetFeatureProcessorFromEntityId or GetMainScene.
Tested with Editor, UI Editor, Material Editor, game launcher.

Signed-off-by: Qing Tao <55564570+VickyAtAZ@users.noreply.github.com>
(cherry picked from commit 8da6bea073)

Gems/Atom/Bootstrap/Code/Source/BootstrapSystemComponent.cpp

@@ -272,6 +272,7 @@ namespace AZ
 
                 // Create and register a scene with all available feature processors
                 RPI::SceneDescriptor sceneDesc;
+                sceneDesc.m_nameId = AZ::Name("Main");
                 AZ::RPI::ScenePtr atomScene = RPI::Scene::CreateScene(sceneDesc);
                 atomScene->EnableAllFeatureProcessors();
                 atomScene->Activate();