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.cc b/common/dynamic_partition_control_stub.cc
index 974cd1b..cc36c5c 100644
--- a/common/dynamic_partition_control_stub.cc
+++ b/common/dynamic_partition_control_stub.cc
@@ -52,4 +52,8 @@
   return true;
 }
 
+ErrorCode DynamicPartitionControlStub::CleanupSuccessfulUpdate() {
+  return ErrorCode::kError;
+}
+
 }  // namespace chromeos_update_engine