Undo P4 check before spawning thread and fix usage of new AzToolsApplication trait (#4894)

Signed-off-by: Steve Pham <spham@amazon.com>

Gems/Atom/Tools/AtomToolsFramework/Code/Source/Application/AtomToolsApplication.cpp

@@ -36,6 +36,8 @@
 #include <AzToolsFramework/UI/UICore/QTreeViewStateSaver.hxx>
 #include <AzToolsFramework/UI/UICore/QWidgetSavedState.h>
 
+#include "AtomToolsFramework_Traits_Platform.h"
+
 AZ_PUSH_DISABLE_WARNING(4251 4800, "-Wunknown-warning-option") // disable warnings spawned by QT
 #include <QMessageBox>
 #include <QObject>
@@ -217,7 +219,7 @@ namespace AtomToolsFramework
         AzFramework::AssetSystemRequestBus::Broadcast(&AzFramework::AssetSystem::AssetSystemRequests::StartDisconnectingAssetProcessor);
 
 #if AZ_TRAIT_ATOMTOOLSFRAMEWORK_SKIP_APP_DESTROY
-        _exit(0);
+        ::_exit(0);
 #else
         Base::Destroy();
 #endif