Mutex: Note that STL is generally prefered

This class was designed prior to Android having usable C++11
support.  With that support now in place, we prefer people to
start using the STL threading classes, unless the code needs
to build on Win32.

Change-Id: If1b4d9bdfcb1e65824909376f022842bab9653d6

include/utils/Mutex.h

@@ -35,6 +35,10 @@ namespace android {
 class Condition;
 
 /*
+ * NOTE: This class is for code that builds on Win32.  Its usage is
+ * deprecated for code which doesn't build for Win32.  New code which
+ * doesn't build for Win32 should use std::mutex and std::lock_guard instead.
+ *
  * Simple mutex class.  The implementation is system-dependent.
  *
  * The mutex must be unlocked by the thread that locked it.  They are not