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/Extensions.cpp b/libs/hwui/Extensions.cpp
index 84e7e65..8352c3f 100644
--- a/libs/hwui/Extensions.cpp
+++ b/libs/hwui/Extensions.cpp
@@ -66,7 +66,7 @@
     findExtensions(eglQueryString(eglGetCurrentDisplay(), EGL_EXTENSIONS), mEglExtensionList);
 
     char property[PROPERTY_VALUE_MAX];
-    if (property_get(PROPERTY_DEBUG_NV_PROFILING, property, NULL) > 0) {
+    if (property_get(PROPERTY_DEBUG_NV_PROFILING, property, nullptr) > 0) {
         mHasNvSystemTime = !strcmp(property, "true") && hasEglExtension("EGL_NV_system_time");
     } else {
         mHasNvSystemTime = false;