Set default updater URL and version per platform.

Chrome OS and Brillo devices require different production update URL
and updater string ID sent to Omaha. This patch moves these constants
to a platform-dependent file with the definition of the constants.

Bug: brillo:587,brillo:588
Test: FEATURES=test emerge-link update_engine; `mma`; deployed on a dragonboard.

Change-Id: Ibe3b30c32c2fb2caf3ed10a4198272dc13a44d12
diff --git a/omaha_response_handler_action_unittest.cc b/omaha_response_handler_action_unittest.cc
index 20d0166..5e14511 100644
--- a/omaha_response_handler_action_unittest.cc
+++ b/omaha_response_handler_action_unittest.cc
@@ -23,6 +23,7 @@
 #include "update_engine/constants.h"
 #include "update_engine/fake_system_state.h"
 #include "update_engine/mock_payload_state.h"
+#include "update_engine/platform_constants.h"
 #include "update_engine/test_utils.h"
 #include "update_engine/utils.h"
 
@@ -408,7 +409,7 @@
   // We're using a real OmahaRequestParams object here so we can't mock
   // IsUpdateUrlOfficial(), but setting the update URL to the AutoUpdate test
   // server will cause IsUpdateUrlOfficial() to return true.
-  params.set_update_url(kAUTestOmahaUrl);
+  params.set_update_url(constants::kOmahaDefaultAUTestURL);
   fake_system_state_.set_request_params(&params);
 
   EXPECT_CALL(*fake_system_state_.mock_payload_state(),