AU: Try delta updates first, then full updates

Also, some bug fixes.

Review URL: http://codereview.chromium.org/492008
diff --git a/postinstall_runner_action_unittest.cc b/postinstall_runner_action_unittest.cc
index c43cb8a..5bb515f 100644
--- a/postinstall_runner_action_unittest.cc
+++ b/postinstall_runner_action_unittest.cc
@@ -56,7 +56,8 @@
   ASSERT_EQ(0, getuid()) << "Run me as root. Ideally don't run other tests "
                          << "as root, tho.";
 
-  const string mountpoint(utils::kStatefulPartition + "/au_destination");
+  const string mountpoint(string(utils::kStatefulPartition) +
+                          "/au_destination");
 
   string cwd;
   {