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.cc b/postinstall_runner_action.cc
index 4122860..98de1bd 100644
--- a/postinstall_runner_action.cc
+++ b/postinstall_runner_action.cc
@@ -12,7 +12,7 @@
 using std::string;
 
 namespace {
-const string kMountPath(utils::kStatefulPartition + "/au_destination");
+const string kMountPath(string(utils::kStatefulPartition) + "/au_destination");
 const string kPostinstallScript("/postinst");
 }