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/AssetAtlas.h b/libs/hwui/AssetAtlas.h
index dba9e62..1772eff 100644
--- a/libs/hwui/AssetAtlas.h
+++ b/libs/hwui/AssetAtlas.h
@@ -106,7 +106,7 @@
         friend class AssetAtlas;
     };
 
-    AssetAtlas(): mTexture(NULL), mImage(NULL),
+    AssetAtlas(): mTexture(nullptr), mImage(nullptr),
             mBlendKey(true), mOpaqueKey(false) { }
     ~AssetAtlas() { terminate(); }