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/binder/IMemory.cpp b/libs/binder/IMemory.cpp
index cd2451a..07cb41a 100644
--- a/libs/binder/IMemory.cpp
+++ b/libs/binder/IMemory.cpp
@@ -246,9 +246,7 @@
                 if (VERBOSE) {
                     ALOGD("UNMAPPING binder=%p, heap=%p, size=%d, fd=%d",
                             binder.get(), this, mSize, mHeapId);
-                    CallStack stack;
-                    stack.update();
-                    stack.dump("callstack");
+                    CallStack stack(LOG_TAG);
                 }
 
                 munmap(mBase, mSize);