improved CallStack a bit

- added a ctor that updates and dumps the stack immediately
- added a "logtag" parameter to dump()

Change-Id: Ie51c256071d282591752243bdb4f68cf9ff8829d
diff --git a/libs/ui/Region.cpp b/libs/ui/Region.cpp
index 1a613f8..488fba3 100644
--- a/libs/ui/Region.cpp
+++ b/libs/ui/Region.cpp
@@ -530,9 +530,7 @@
     }
     if (result == false && !silent) {
         reg.dump(name);
-        CallStack stack;
-        stack.update();
-        stack.dump("");
+        CallStack stack(LOG_TAG);
     }
     return result;
 }