Call SnapshotManager::FinishedSnapshotWrites

... when the update has finished. This allows SnapshotManager
to conclude the update and prepare for reboot.

Test: builds
Test: apply OTA on Virtual A/B device, then check
/metadata/ota/snapshot-boot exists, then reboot
Bug: 138816109
Change-Id: I2a4699865b09358ef018313bed64e34617a78e3c
diff --git a/mock_dynamic_partition_control.h b/mock_dynamic_partition_control.h
index d96432b..1af6cfe 100644
--- a/mock_dynamic_partition_control.h
+++ b/mock_dynamic_partition_control.h
@@ -48,6 +48,7 @@
                bool(uint32_t, uint32_t, const DeltaArchiveManifest&));
   MOCK_METHOD1(GetSuperPartitionName, std::string(uint32_t));
   MOCK_METHOD0(GetVirtualAbFeatureFlag, FeatureFlag());
+  MOCK_METHOD0(FinishUpdate, bool());
 };
 
 class MockDynamicPartitionControlAndroid
@@ -75,6 +76,7 @@
   MOCK_METHOD0(GetDynamicPartitionsFeatureFlag, FeatureFlag());
   MOCK_METHOD1(GetSuperPartitionName, std::string(uint32_t));
   MOCK_METHOD0(GetVirtualAbFeatureFlag, FeatureFlag());
+  MOCK_METHOD0(FinishUpdate, bool());
 };
 
 }  // namespace chromeos_update_engine