Properly shut down the exchange manager when shutting down system sta…

…te. (#10359)

The leak that caused us to not do this before seems to be fixed with
the exchange lifetime work that has happened since.

Fixes #6668

src/controller/CHIPDeviceControllerFactory.cpp

@@ -235,11 +235,10 @@ CHIP_ERROR DeviceControllerSystemState::Shutdown()
     ReturnErrorOnFailure(DeviceLayer::PlatformMgr().Shutdown());
 #endif
 
-    // TODO(#6668): Some exchange has leak, shutting down ExchangeManager will cause a assert fail.
-    // if (mExchangeMgr != nullptr)
-    // {
-    //     mExchangeMgr->Shutdown();
-    // }
+    if (mExchangeMgr != nullptr)
+    {
+        mExchangeMgr->Shutdown();
+    }
     if (mSessionMgr != nullptr)
     {
         mSessionMgr->Shutdown();