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