Use Common::Flag and Common::Event when possible

Replaces old and simple usages of std::atomic<bool> with Common::Flag
(which was introduced after the initial usage), so it's clear that
the variable is a flag and because Common::Flag is well tested.

This also replaces the ready logic in WiimoteReal with Common::Event
since it was basically just unnecessarily reimplementing Common::Event.

Source/Core/AudioCommon/AOSoundStream.h

@@ -4,7 +4,6 @@
 
 #pragma once
 
-#include <atomic>
 #include <mutex>
 #include <thread>
 
@@ -20,7 +19,7 @@ class AOSound final : public SoundStream
 {
 #if defined(HAVE_AO) && HAVE_AO
   std::thread thread;
-  std::atomic<bool> m_run_thread;
+  Common::Flag m_run_thread;
   std::mutex soundCriticalSection;
   Common::Event soundSyncEvent;