Fix AssertNonMovableStringClass().

Use standard thread state transition.

Test: testrunner.py --host --jit --gcstress -t 157
Bug: 126246773
Change-Id: I496ede614508f9c84692c4fa549153025c6ef9b4
diff --git a/compiler/optimizing/intrinsics.cc b/compiler/optimizing/intrinsics.cc
index 2de0f0c..d940105 100644
--- a/compiler/optimizing/intrinsics.cc
+++ b/compiler/optimizing/intrinsics.cc
@@ -365,8 +365,7 @@
 
 void IntrinsicVisitor::AssertNonMovableStringClass() {
   if (kIsDebugBuild) {
-    Thread* const self = Thread::Current();
-    ReaderMutexLock mu(self, *Locks::mutator_lock_);
+    ScopedObjectAccess soa(Thread::Current());
     ObjPtr<mirror::Class> string_class = GetClassRoot<art::mirror::String>();
     CHECK(!art::Runtime::Current()->GetHeap()->IsMovableObject(string_class));
   }