thread: Enforce inine rules

Bug: 438841609
Test: Build passed.
Flag: EXEMPT, no-op change.
Change-Id: I9557b056ca663a209eff20fc926e99ad86e066ad
diff --git a/bluetooth/bluetooth_hal/extensions/thread/thread_handler.cc b/bluetooth/bluetooth_hal/extensions/thread/thread_handler.cc
index ee3e79c..65251e5 100644
--- a/bluetooth/bluetooth_hal/extensions/thread/thread_handler.cc
+++ b/bluetooth/bluetooth_hal/extensions/thread/thread_handler.cc
@@ -32,9 +32,6 @@
 using ::bluetooth_hal::hci::MonitorMode;
 using ::bluetooth_hal::thread::ThreadDaemon;
 
-std::mutex ThreadHandler::mutex_;
-std::unique_ptr<ThreadHandler> ThreadHandler::handler_ = nullptr;
-
 ThreadHandler::ThreadHandler() {
   thread_daemon_ = std::make_unique<ThreadDaemon>(
       [this](const ::bluetooth_hal::hci::HalPacket& packet) {
diff --git a/bluetooth/bluetooth_hal/include/public/bluetooth_hal/extensions/thread/thread_handler.h b/bluetooth/bluetooth_hal/include/public/bluetooth_hal/extensions/thread/thread_handler.h
index 789987a..6b1bb24 100644
--- a/bluetooth/bluetooth_hal/include/public/bluetooth_hal/extensions/thread/thread_handler.h
+++ b/bluetooth/bluetooth_hal/include/public/bluetooth_hal/extensions/thread/thread_handler.h
@@ -155,8 +155,8 @@
 
  private:
   std::unique_ptr<ThreadDaemon> thread_daemon_;
-  static std::mutex mutex_;
-  static std::unique_ptr<ThreadHandler> handler_;
+  static inline std::mutex mutex_;
+  static inline std::unique_ptr<ThreadHandler> handler_{nullptr};
   ::bluetooth_hal::hci::HciThreadMonitor thread_data_monitor_;
 };