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/licm_test.cc b/compiler/optimizing/licm_test.cc
index 2bb769a..9ad003c 100644
--- a/compiler/optimizing/licm_test.cc
+++ b/compiler/optimizing/licm_test.cc
@@ -107,7 +107,7 @@
   BuildLoop();
 
   // Populate the loop with instructions: set/get field with different types.
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
   HInstruction* get_field = new (&allocator_) HInstanceFieldGet(parameter_,
                                                                 Primitive::kPrimLong,
                                                                 MemberOffset(10),
@@ -134,7 +134,7 @@
   BuildLoop();
 
   // Populate the loop with instructions: set/get field with same types.
-  NullHandle<mirror::DexCache> dex_cache;
+  ScopedNullHandle<mirror::DexCache> dex_cache;
   HInstruction* get_field = new (&allocator_) HInstanceFieldGet(parameter_,
                                                                 Primitive::kPrimLong,
                                                                 MemberOffset(10),