Move CallStack to libbacktrace.

Fix a small bug in the Printer for strings that didn't properly
prepend the prefix.

Change-Id: I33c63841ef6e07728ab3195886539d82b38ee19a
diff --git a/libutils/Printer.cpp b/libutils/Printer.cpp
index ac729e0..263e740 100644
--- a/libutils/Printer.cpp
+++ b/libutils/Printer.cpp
@@ -145,6 +145,7 @@
         return;
     }
 
+    mTarget->append(mPrefix);
     mTarget->append(string);
     mTarget->append("\n");
 }