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

libs/hwui/Matrix.h

diff --git a/libs/hwui/Matrix.h b/libs/hwui/Matrix.h
index 1c5c578..a760135 100644
--- a/libs/hwui/Matrix.h
+++ b/libs/hwui/Matrix.h
@@ -210,7 +210,7 @@
 
     void decomposeScale(float& sx, float& sy) const;
 
-    void dump(const char* label = NULL) const;
+    void dump(const char* label = nullptr) const;
 
     static const Matrix4& identity();