Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/Outline.h b/libs/hwui/Outline.h
index 6dacd5ed..5e9b213 100644
--- a/libs/hwui/Outline.h
+++ b/libs/hwui/Outline.h
@@ -95,7 +95,7 @@
     }
 
     const SkPath* getPath() const {
-        if (mType == kOutlineType_None || mType == kOutlineType_Empty) return NULL;
+        if (mType == kOutlineType_None || mType == kOutlineType_Empty) return nullptr;
 
         return &mPath;
     }