Shorten the namespace and re-normalize include guards.

Change-Id: I93af4c1f19edf31098abd20ded1fcca6c5d83854
diff --git a/src/casts.h b/src/casts.h
index f9f3c6b..15852cf 100644
--- a/src/casts.h
+++ b/src/casts.h
@@ -6,8 +6,7 @@
 #include <string.h>
 #include "src/macros.h"
 
-namespace android {
-namespace runtime {
+namespace art {
 
 template <class Dest, class Source>
 inline Dest bit_cast(const Source& source) {
@@ -19,6 +18,6 @@
   return dest;
 }
 
-} }  // namespace android::runtime
+}  // namespace art
 
 #endif  // ART_SRC_CASTS_H_