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/Rect.h b/libs/hwui/Rect.h
index 13265a9..b046b6f 100644
--- a/libs/hwui/Rect.h
+++ b/libs/hwui/Rect.h
@@ -248,7 +248,7 @@
         bottom = fmaxf(bottom, y);
     }
 
-    void dump(const char* label = NULL) const {
+    void dump(const char* label = nullptr) const {
         ALOGD("%s[l=%f t=%f r=%f b=%f]", label ? label : "Rect", left, top, right, bottom);
     }