Add overrides and switch to nullptr keyword for all files

Adds remaining missing overrides and nullptr usages, missed due to
an extreme failure in tool usage.

Change-Id: I56abd72975a3999ad13330003c348db40f59aebf
diff --git a/libs/hwui/GradientCache.cpp b/libs/hwui/GradientCache.cpp
index 06d234c..0987d9b 100644
--- a/libs/hwui/GradientCache.cpp
+++ b/libs/hwui/GradientCache.cpp
@@ -66,7 +66,7 @@
         mCache(LruCache<GradientCacheEntry, Texture*>::kUnlimitedCapacity),
         mSize(0), mMaxSize(MB(DEFAULT_GRADIENT_CACHE_SIZE)) {
     char property[PROPERTY_VALUE_MAX];
-    if (property_get(PROPERTY_GRADIENT_CACHE_SIZE, property, NULL) > 0) {
+    if (property_get(PROPERTY_GRADIENT_CACHE_SIZE, property, nullptr) > 0) {
         INIT_LOGD("  Setting gradient cache size to %sMB", property);
         setMaxSize(MB(atof(property)));
     } else {