Add DynamicPartitionControlInterface::CleanupSuccessfulUpdate

This is a wrapper over SnapshotManager::WaitForMerge. It waits until the
previous update is merged, then return.

Bug: 138808328
Test: manual with update_engine_client

Change-Id: If44854810f37dd959ffdf3f62f26528867a71fc8
diff --git a/common/dynamic_partition_control_stub.h b/common/dynamic_partition_control_stub.h
index 09990a7..02575a1 100644
--- a/common/dynamic_partition_control_stub.h
+++ b/common/dynamic_partition_control_stub.h
@@ -39,6 +39,7 @@
                                   uint64_t* required_size) override;
 
   bool FinishUpdate() override;
+  ErrorCode CleanupSuccessfulUpdate() override;
 };
 
 }  // namespace chromeos_update_engine