AU: Try delta updates first, then full updates

Also, some bug fixes.

Review URL: http://codereview.chromium.org/492008
diff --git a/utils.cc b/utils.cc
index 9fa0906..69a59cc 100644
--- a/utils.cc
+++ b/utils.cc
@@ -257,7 +257,7 @@
   return true;
 }
 
-const string kStatefulPartition = "/mnt/stateful_partition";
+const char* const kStatefulPartition = "/mnt/stateful_partition";
 
 }  // namespace utils