AU: Try delta updates first, then full updates

Also, some bug fixes.

Review URL: http://codereview.chromium.org/492008
diff --git a/test_utils.h b/test_utils.h
index 6274a22..cd17ec1 100644
--- a/test_utils.h
+++ b/test_utils.h
@@ -88,7 +88,7 @@
   0xbe, 0x9f, 0xa3, 0x5d
 };
 
-const string kMountPath("/tmp/UpdateEngineTests_mnt");
+const char* const kMountPath = "/tmp/UpdateEngineTests_mnt";
 }  // namespace {}
 
 // Creates an ext image with some files in it. The paths creates are