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/TextureCache.h b/libs/hwui/TextureCache.h
index d7d51a1..0e33e4c 100644
--- a/libs/hwui/TextureCache.h
+++ b/libs/hwui/TextureCache.h
@@ -62,7 +62,7 @@
      * Used as a callback when an entry is removed from the cache.
      * Do not invoke directly.
      */
-    void operator()(uint32_t&, Texture*& texture);
+    void operator()(uint32_t&, Texture*& texture) override;
 
     /**
      * Resets all Textures to not be marked as in use