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/Layer.cpp b/libs/hwui/Layer.cpp
index 9053c12..fc3118a 100644
--- a/libs/hwui/Layer.cpp
+++ b/libs/hwui/Layer.cpp
@@ -20,6 +20,8 @@
 #include "utils/Color.h"
 #include "utils/MathUtils.h"
 
+#include <SkBlendMode.h>
+
 #include <log/log.h>
 
 namespace android {