Fix <android-base/*.h> header guards.

I've been meaning to do this for a while, and it came up on the bug below
that there have been conflicts with similar-named files in <base/*.h>,
so let's rule out one possible explanation.

Bug: http://b/27804373
Change-Id: I69e5d52b6260c573c308513420aee0e281426bd4
diff --git a/include/android-base/test_utils.h b/include/android-base/test_utils.h
index 3f6872c..4ea3c8e 100644
--- a/include/android-base/test_utils.h
+++ b/include/android-base/test_utils.h
@@ -14,8 +14,8 @@
  * limitations under the License.
  */
 
-#ifndef TEST_UTILS_H
-#define TEST_UTILS_H
+#ifndef ANDROID_BASE_TEST_UTILS_H
+#define ANDROID_BASE_TEST_UTILS_H
 
 #include <string>
 
@@ -48,4 +48,4 @@
   DISALLOW_COPY_AND_ASSIGN(TemporaryDir);
 };
 
-#endif // TEST_UTILS_H
+#endif  // ANDROID_BASE_TEST_UTILS_H