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/trace.cc b/runtime/trace.cc
index 5a44947..e2743ce 100644
--- a/runtime/trace.cc
+++ b/runtime/trace.cc
@@ -73,6 +73,8 @@
 
  private:
   std::vector<ArtMethod*>* const method_trace_;
+
+  DISALLOW_COPY_AND_ASSIGN(BuildStackTraceVisitor);
 };
 
 static const char     kTraceTokenChar             = '*';