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/Properties.h b/libs/hwui/Properties.h
index 7b9459a..a0312e1 100644
--- a/libs/hwui/Properties.h
+++ b/libs/hwui/Properties.h
@@ -255,7 +255,7 @@
 
 static inline DebugLevel readDebugLevel() {
     char property[PROPERTY_VALUE_MAX];
-    if (property_get(PROPERTY_DEBUG, property, NULL) > 0) {
+    if (property_get(PROPERTY_DEBUG, property, nullptr) > 0) {
         return (DebugLevel) atoi(property);
     }
     return kDebugDisabled;