Add cli option to enable multi-threaded compression

Test: update_device.py --enable-threading, make sure threading is being
turned on

Change-Id: I1d0e5ab5c2e65cc48b5126fb186f770b68188351
diff --git a/common/constants.h b/common/constants.h
index 11be740..36c4476 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -179,6 +179,10 @@
 // Set Virtual AB Compression's compression algorithm to "none", but still use
 // userspace snapshots and snapuserd for update installation.
 static constexpr const auto& kPayloadDisableVABC = "DISABLE_VABC";
+// Enable multi-threaded compression for VABC
+static constexpr const auto& kPayloadEnableThreading = "ENABLE_THREADING";
+// Enable batched writes for VABC
+static constexpr const auto& kPayloadBatchedWrites = "BATCHED_WRITES";
 
 // Max retry count for download
 static constexpr const auto& kPayloadDownloadRetry = "DOWNLOAD_RETRY";