Add missing includes of SkBlendMode.h

It's being removed from SkPaint.h in https://skia-review.googlesource.com/c/skia/+/611077
and this was all the places I could find in frameworks/base.

We might need another CL for a few other Android repos.

Change-Id: I88d31166ed90356c4c52fbcde6bec6c1655b7cf2
diff --git a/libs/hwui/tests/unit/CanvasOpTests.cpp b/libs/hwui/tests/unit/CanvasOpTests.cpp
index d2b1ef9..1f6edf3 100644
--- a/libs/hwui/tests/unit/CanvasOpTests.cpp
+++ b/libs/hwui/tests/unit/CanvasOpTests.cpp
@@ -23,6 +23,7 @@
 
 #include <tests/common/CallCountingCanvas.h>
 
+#include "SkBlendMode.h"
 #include "SkBitmap.h"
 #include "SkCanvas.h"
 #include "SkColor.h"