Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/PatchCache.h b/libs/hwui/PatchCache.h
index 1e40997..4cb5338 100644
--- a/libs/hwui/PatchCache.h
+++ b/libs/hwui/PatchCache.h
@@ -91,7 +91,7 @@
 
 private:
     struct PatchDescription {
-        PatchDescription(): mPatch(NULL), mBitmapWidth(0), mBitmapHeight(0),
+        PatchDescription(): mPatch(nullptr), mBitmapWidth(0), mBitmapHeight(0),
                 mPixelWidth(0), mPixelHeight(0) {
         }
 
@@ -146,7 +146,7 @@
      * to track available regions of memory in the VBO.
      */
     struct BufferBlock {
-        BufferBlock(uint32_t offset, uint32_t size): offset(offset), size(size), next(NULL) {
+        BufferBlock(uint32_t offset, uint32_t size): offset(offset), size(size), next(nullptr) {
         }
 
         uint32_t offset;