Move CallStack to libbacktrace.

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

(cherry picked from commit 9b0e074c6d38143e01616404a08b0c7aa992f3c3)

Change-Id: I78bfa3f76864c34f33fb439bf20dfc85616f1077
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");
 }