Cleanup thread access in StackVisitor

Adds method StackVisitor::GetThread to give access to the visited
Thread* so we no longer need to copy that pointer in subclasses.

Also adds a few missing const and DISALLOW_COPY_AND_ASSIGN.

Change-Id: I57649ee7742ef4ef1e01447ac2fbb66f977b22eb
diff --git a/runtime/stack.h b/runtime/stack.h
index 2562738..5bbf003 100644
--- a/runtime/stack.h
+++ b/runtime/stack.h
@@ -441,6 +441,10 @@
   void WalkStack(bool include_transitions = false)
       SHARED_REQUIRES(Locks::mutator_lock_);
 
+  Thread* GetThread() const {
+    return thread_;
+  }
+
   ArtMethod* GetMethod() const SHARED_REQUIRES(Locks::mutator_lock_);
 
   bool IsShadowFrame() const {