Fix shutdown ordering in DeviceController. (#7430)

Before this fix we would tear down some things (importantly the secure
session manager) before starting shutdown of the network layer.  This
would lead to a window of time during which we can still receive
messages while in a partially torn down state, which would leave to
crashes.

Moving network layer shutdown, and in particular platform manager
shutdown, to be first in the shutdown sequence ensures this can't
happen by shutting down the message processing thread before we tear
down any other state.

Fixes #7297

src/controller/CHIPDeviceController.cpp

@@ -292,6 +292,20 @@ CHIP_ERROR DeviceController::Shutdown()
 
     ChipLogDetail(Controller, "Shutting down the controller");
 
+#if CONFIG_DEVICE_LAYER
+    // Start by shutting down the PlatformManager.  This will ensure, with
+    // reasonable synchronization, that we stop processing of incoming messages
+    // before doing any other shutdown work.  Otherwise we can end up trying to
+    // process incoming messages in a partially shut down state, which is not
+    // great at all.
+    ReturnErrorOnFailure(DeviceLayer::PlatformMgr().Shutdown());
+#else
+    mInetLayer->Shutdown();
+    mSystemLayer->Shutdown();
+    chip::Platform::Delete(mInetLayer);
+    chip::Platform::Delete(mSystemLayer);
+#endif // CONFIG_DEVICE_LAYER
+
     mState = State::NotInitialized;
 
     // TODO(#6668): Some exchange has leak, shutting down ExchangeManager will cause a assert fail.
@@ -304,15 +318,6 @@ CHIP_ERROR DeviceController::Shutdown()
         mSessionMgr->Shutdown();
     }
 
-#if CONFIG_DEVICE_LAYER
-    ReturnErrorOnFailure(DeviceLayer::PlatformMgr().Shutdown());
-#else
-    mInetLayer->Shutdown();
-    mSystemLayer->Shutdown();
-    chip::Platform::Delete(mInetLayer);
-    chip::Platform::Delete(mSystemLayer);
-#endif // CONFIG_DEVICE_LAYER
-
     mSystemLayer     = nullptr;
     mInetLayer       = nullptr;
     mStorageDelegate = nullptr;