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/compiler/optimizing/register_allocator_test.cc b/compiler/optimizing/register_allocator_test.cc
index 080f970..8706854 100644
--- a/compiler/optimizing/register_allocator_test.cc
+++ b/compiler/optimizing/register_allocator_test.cc
@@ -472,7 +472,7 @@
                                   HInstruction** input2) {
   HGraph* graph = CreateGraph(allocator);
   HBasicBlock* entry = new (allocator) HBasicBlock(graph);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
   graph->AddBlock(entry);
   graph->SetEntryBlock(entry);
   HInstruction* parameter = new (allocator) HParameterValue(
@@ -624,7 +624,7 @@
                                 HInstruction** field,
                                 HInstruction** ret) {
   HGraph* graph = CreateGraph(allocator);
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
   HBasicBlock* entry = new (allocator) HBasicBlock(graph);
   graph->AddBlock(entry);
   graph->SetEntryBlock(entry);