Remove 64-bit android_atomic uses from StateQueue.

Use stdatomic.h instead.  We're trying to remove android_atomic use
wherever possible.  The 64-bit uses seem easiest to remove first.
This cleans up the code, though not as much as C++ <atomic> would,
if it worked everywhere.

Change-Id: I3c29bdbd5915cb9d47118834a3a742fe296cf87f
diff --git a/services/audioflinger/StateQueue.h b/services/audioflinger/StateQueue.h
index 9cde642..ef01df7 100644
--- a/services/audioflinger/StateQueue.h
+++ b/services/audioflinger/StateQueue.h
@@ -17,6 +17,8 @@
 #ifndef ANDROID_AUDIO_STATE_QUEUE_H
 #define ANDROID_AUDIO_STATE_QUEUE_H
 
+#include <stdatomic.h>
+
 // The state queue template class was originally driven by this use case / requirements:
 //  There are two threads: a fast mixer, and a normal mixer, and they share state.
 //  The interesting part of the shared state is a set of active fast tracks,
@@ -184,7 +186,7 @@
     T                 mStates[kN];      // written by mutator, read by observer
 
     // "volatile" is meaningless with SMP, but here it indicates that we're using atomic ops
-    volatile const T* mNext; // written by mutator to advance next, read by observer
+    atomic_uintptr_t  mNext; // written by mutator to advance next, read by observer
     volatile const T* mAck;  // written by observer to acknowledge advance of next, read by mutator
 
     // only used by observer