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/utils/PaintUtils.h b/libs/hwui/utils/PaintUtils.h
index 8a4034f..fa0ae03 100644
--- a/libs/hwui/utils/PaintUtils.h
+++ b/libs/hwui/utils/PaintUtils.h
@@ -44,13 +44,13 @@
     // TODO: move to a method on android:Paint? replace with SkPaint::nothingToDraw()?
     static inline bool paintWillNotDrawText(const SkPaint& paint) {
         return paint.getAlpha() == 0
-                && paint.getLooper() == NULL
+                && paint.getLooper() == nullptr
                 && !paint.getColorFilter()
                 && getXfermode(paint.getXfermode()) == SkXfermode::kSrcOver_Mode;
     }
 
     static bool isBlendedColorFilter(const SkColorFilter* filter) {
-        if (filter == NULL) {
+        if (filter == nullptr) {
             return false;
         }
         return (filter->getFlags() & SkColorFilter::kAlphaUnchanged_Flag) == 0;