HCI Layer: Register CommandComplete and CommandStatus on Start()

Module.GetHandler() works only after a Module is started. Move
RegisterEventHandler() for CommandComplete and CommandStatus to Start().

Test: atest bluetooth_test_gd --host
Change-Id: I7331c8ddb8d61d697aa4480e7c591c3817d21afa
diff --git a/gd/module.h b/gd/module.h
index 50f231b..6b2ed18 100644
--- a/gd/module.h
+++ b/gd/module.h
@@ -86,7 +86,7 @@
  private:
   Module* GetDependency(const ModuleFactory* module) const;
 
-  ::bluetooth::os::Handler* handler_;
+  ::bluetooth::os::Handler* handler_ = nullptr;
   ModuleList dependencies_;
   ModuleRegistry* registry_;
 };