Remove incorrect android_atomic_...64 use.

Change the mExtras field in Binder.h to be a stdatomic.h atomic
value, and replace references to it with proper stdatomic.h calls.
This removes one of a small number of remaining 64 bit
android_atomic references.  It also replaces the erroneously
non-atomic read accesses to mExtras.

It would be better if this used the C++11 <atomic> facility,
but we don't quite have that yet.

Bug: 16513433
Change-Id: Ibabb88d05025187ee1ce6c7f1aa670b133a547f8
diff --git a/include/binder/Binder.h b/include/binder/Binder.h
index ba3ac4b..16f2c4a 100644
--- a/include/binder/Binder.h
+++ b/include/binder/Binder.h
@@ -17,6 +17,7 @@
 #ifndef ANDROID_BINDER_H
 #define ANDROID_BINDER_H
 
+#include <stdatomic.h>
 #include <binder/IBinder.h>
 
 // ---------------------------------------------------------------------------
@@ -69,7 +70,7 @@
 
     class Extras;
 
-            Extras*     mExtras;
+    atomic_uintptr_t    mExtras;  // should be atomic<Extras *>
             void*       mReserved0;
 };