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/renderthread/DrawFrameTask.cpp b/libs/hwui/renderthread/DrawFrameTask.cpp
index 97b31a9..4d8a469 100644
--- a/libs/hwui/renderthread/DrawFrameTask.cpp
+++ b/libs/hwui/renderthread/DrawFrameTask.cpp
@@ -32,8 +32,8 @@
 namespace renderthread {
 
 DrawFrameTask::DrawFrameTask()
-        : mRenderThread(NULL)
-        , mContext(NULL)
+        : mRenderThread(nullptr)
+        , mContext(nullptr)
         , mFrameTimeNanos(0)
         , mRecordDurationNanos(0)
         , mDensity(1.0f) // safe enough default