Don't define DISALLOW_COPY_AND_ASSIGN if already defined.

Required by:
https://android-review.googlesource.com/#/c/113053/

Change-Id: I0226afffc738a4b7c162a0d467513a35d62dbfdc
diff --git a/runtime/base/macros.h b/runtime/base/macros.h
index 90cf951..d297439 100644
--- a/runtime/base/macros.h
+++ b/runtime/base/macros.h
@@ -70,9 +70,11 @@
 
 // DISALLOW_COPY_AND_ASSIGN disallows the copy and operator= functions. It goes in the private:
 // declarations in a class.
+#if !defined(DISALLOW_COPY_AND_ASSIGN)
 #define DISALLOW_COPY_AND_ASSIGN(TypeName) \
   TypeName(const TypeName&) = delete;  \
   void operator=(const TypeName&) = delete
+#endif
 
 // A macro to disallow all the implicit constructors, namely the default constructor, copy
 // constructor and operator= functions.