Reapply "Refactor setting an SkPaint onto a hwui Layer."

This reverts commit 90d0c75e94a32fb7d993fae69762820aabc2fcbb.

Change-Id: Ie807761231edf7c848b019931dccbf466208c0be
diff --git a/libs/hwui/Layer.h b/libs/hwui/Layer.h
index ec80e9c..8cc027a 100644
--- a/libs/hwui/Layer.h
+++ b/libs/hwui/Layer.h
@@ -117,7 +117,7 @@
         texture.height = height;
     }
 
-    ANDROID_API void setPaint(SkPaint* paint);
+    ANDROID_API void setPaint(const SkPaint* paint);
 
     inline void setBlend(bool blend) {
         texture.blend = blend;