Shorten the namespace and re-normalize include guards.

Change-Id: I93af4c1f19edf31098abd20ded1fcca6c5d83854
diff --git a/src/memory_region.cc b/src/memory_region.cc
index 1fa5d1f..a10516f 100644
--- a/src/memory_region.cc
+++ b/src/memory_region.cc
@@ -6,8 +6,7 @@
 #include "src/logging.h"
 #include "src/memory_region.h"
 
-namespace android {
-namespace runtime {
+namespace art {
 
 void MemoryRegion::CopyFrom(size_t offset, const MemoryRegion& from) const {
   CHECK_NE(from.pointer(), NULL);
@@ -18,4 +17,4 @@
           from.pointer(), from.size());
 }
 
-} }  // namespace android::runtime
+}  // namespace art