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/Animator.cpp b/libs/hwui/Animator.cpp
index 805f844..512e0e2 100644
--- a/libs/hwui/Animator.cpp
+++ b/libs/hwui/Animator.cpp
@@ -32,7 +32,7 @@
  ************************************************************/
 
 BaseRenderNodeAnimator::BaseRenderNodeAnimator(float finalValue)
-        : mTarget(NULL)
+        : mTarget(nullptr)
         , mFinalValue(finalValue)
         , mDeltaValue(0)
         , mFromValue(0)