buffet: Add DeviceId DBus property

Privetd needs to be notified when GCD ID is changed.

CQ-DEPEND=CL:260681
BUG=brillo:572
TEST=manual, register device using privetd, edit id in buffet.state, restart
             buffet, check if privetd has new id

Change-Id: I590ee22f134261711f78da75a4766043e0867bf5
Reviewed-on: https://chromium-review.googlesource.com/260303
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/device_registration_info_unittest.cc b/buffet/device_registration_info_unittest.cc
index 7909713..0823e77 100644
--- a/buffet/device_registration_info_unittest.cc
+++ b/buffet/device_registration_info_unittest.cc
@@ -221,7 +221,7 @@
                                    mock_callback));
   }
 
-  MOCK_METHOD1(OnRegistrationStatusChange, void(RegistrationStatus));
+  MOCK_METHOD0(OnRegistrationStatusChange, void());
 
   base::DictionaryValue data_;
   std::shared_ptr<MemStorage> storage_;
@@ -385,8 +385,7 @@
   transport_->AddHandler(dev_reg_->GetDeviceURL(),
                          chromeos::http::request_type::kGet,
                          base::Bind(DeviceInfoHandler));
-  std::string id = dev_reg_->GetDeviceId(nullptr);
-  EXPECT_EQ(test_data::kDeviceId, id);
+  EXPECT_EQ(test_data::kDeviceId, dev_reg_->GetDeviceId());
 }
 
 TEST_F(DeviceRegistrationInfoTest, RegisterDevice) {