Shorten the namespace and re-normalize include guards.

Change-Id: I93af4c1f19edf31098abd20ded1fcca6c5d83854
diff --git a/src/assembler_arm.h b/src/assembler_arm.h
index 1ac5a82..8ba9906 100644
--- a/src/assembler_arm.h
+++ b/src/assembler_arm.h
@@ -3,12 +3,11 @@
 #ifndef ART_SRC_ASSEMBLER_ARM_H_
 #define ART_SRC_ASSEMBLER_ARM_H_
 
-namespace android {
-namespace runtime {
+namespace art {
 
 class Assembler {
 };
 
-} }  // namespace android::runtime
+} // namespace art
 
 #endif  // ART_SRC_ASSEMBLER_ARM_H_