Rename NullHandle to ScopedNullHandle

This makes it clearer that is invalid to do things like:
Handle<T> h = ScopedNullHandle<T>();

Bug: 26233305
Change-Id: I6d8f54eae01ec2e901cb7043afa853ea77db79fe
diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index 880d3e0..bc8ba97 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -1383,8 +1383,11 @@
   if (annotation_item == nullptr) {
     return nullptr;
   }
-  mirror::Object* obj = GetAnnotationValue(
-      klass, annotation_item, "value", NullHandle<mirror::Class>(), kDexAnnotationType);
+  mirror::Object* obj = GetAnnotationValue(klass,
+                                           annotation_item,
+                                           "value",
+                                           ScopedNullHandle<mirror::Class>(),
+                                           kDexAnnotationType);
   if (obj == nullptr) {
     return nullptr;
   }
@@ -1410,8 +1413,11 @@
     return nullptr;
   }
   AnnotationValue annotation_value;
-  if (!ProcessAnnotationValue(
-      klass, &annotation, &annotation_value, NullHandle<mirror::Class>(), kAllRaw)) {
+  if (!ProcessAnnotationValue(klass,
+                              &annotation,
+                              &annotation_value,
+                              ScopedNullHandle<mirror::Class>(),
+                              kAllRaw)) {
     return nullptr;
   }
   if (annotation_value.type_ != kDexAnnotationMethod) {
@@ -1439,7 +1445,7 @@
     return nullptr;
   }
   return GetAnnotationValue(
-      klass, annotation_item, "value", NullHandle<mirror::Class>(), kDexAnnotationMethod);
+      klass, annotation_item, "value", ScopedNullHandle<mirror::Class>(), kDexAnnotationMethod);
 }
 
 bool DexFile::GetInnerClass(Handle<mirror::Class> klass, mirror::String** name) const {
@@ -1457,8 +1463,11 @@
     return false;
   }
   AnnotationValue annotation_value;
-  if (!ProcessAnnotationValue(
-      klass, &annotation, &annotation_value, NullHandle<mirror::Class>(), kAllObjects)) {
+  if (!ProcessAnnotationValue(klass,
+                              &annotation,
+                              &annotation_value,
+                              ScopedNullHandle<mirror::Class>(),
+                              kAllObjects)) {
     return false;
   }
   if (annotation_value.type_ != kDexAnnotationNull &&
@@ -1484,8 +1493,11 @@
     return false;
   }
   AnnotationValue annotation_value;
-  if (!ProcessAnnotationValue(
-      klass, &annotation, &annotation_value, NullHandle<mirror::Class>(), kAllRaw)) {
+  if (!ProcessAnnotationValue(klass,
+                              &annotation,
+                              &annotation_value,
+                              ScopedNullHandle<mirror::Class>(),
+                              kAllRaw)) {
     return false;
   }
   if (annotation_value.type_ != kDexAnnotationInt) {