buffet: CheckDeviceRegistered works offline

CheckDeviceRegistered previously worked differently if buffet had never
been online vs if it just went offline. Basically it should just care if
the device_reg_info file has a registration configured. Specifically
it shouldn't try to refresh the access_token.

BUG=brillo:785
TEST=FEATURES=test emerge-$BOARD buffet

Change-Id: I7da25f7e9507a131a37fd16efeb2e8e7352985b8
Reviewed-on: https://chromium-review.googlesource.com/264792
Reviewed-by: Nathan Bullock <nathanbullock@google.com>
Tested-by: Nathan Bullock <nathanbullock@google.com>
Commit-Queue: Nathan Bullock <nathanbullock@google.com>
diff --git a/buffet/manager.cc b/buffet/manager.cc
index 897c8a4..33b1763 100644
--- a/buffet/manager.cc
+++ b/buffet/manager.cc
@@ -83,7 +83,7 @@
 void Manager::CheckDeviceRegistered(DBusMethodResponse<std::string> response) {
   LOG(INFO) << "Received call to Manager.CheckDeviceRegistered()";
   chromeos::ErrorPtr error;
-  bool registered = device_info_->CheckRegistration(&error);
+  bool registered = device_info_->HaveRegistrationCredentials(&error);
   // If it fails due to any reason other than 'device not registered',
   // treat it as a real error and report it to the caller.
   if (!registered &&