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/FontRenderer.h b/libs/hwui/FontRenderer.h
index e11128c..668ee64 100644
--- a/libs/hwui/FontRenderer.h
+++ b/libs/hwui/FontRenderer.h
@@ -66,7 +66,7 @@
     }
     ~TextSetupFunctor() { }
 
-    status_t operator ()(int what, void* data);
+    status_t operator ()(int what, void* data) override;
 
     OpenGLRenderer* renderer;
     float x;