Add a LOG_TAG for android.graphics.Graphics JNI.

It occasionally logs when it fails an allocation but wasn't
defining one before.

Change-Id: Ifc41addc870eb126616ad44465638423d51568d9
diff --git a/core/jni/android/graphics/Graphics.cpp b/core/jni/android/graphics/Graphics.cpp
index 01aad93..5659ba2 100644
--- a/core/jni/android/graphics/Graphics.cpp
+++ b/core/jni/android/graphics/Graphics.cpp
@@ -1,3 +1,5 @@
+#define LOG_TAG "GraphicsJNI"
+
 #include "jni.h"
 #include "GraphicsJNI.h"
 #include "NIOBuffer.h"