Track bytes received across multiple update files

When downloading the packages that comprise a multi-package or multi-app
update, the UpdateAttempter receives BytesReceived() callbacks with
bytes_received resetting to 0 for each file.  This causes the progress
calculations to be incorrect.

This change tracks the total of the previously downloaded packages
within the DownloadAction, so that it properly tracks.  Resumed
downloads will jump ahead over skipped data, when the payload is
incremented.

Bug:  65451460
Tests: Added unit tests to directly test the accumulation and the the
transition from the previous state to UpdateStatus::DOWNLOADING when the
first bytes are received.

Change-Id: I3b540df16b9a664b09f53ee3ec962e2cbc8adf1b
(cherry picked from commit d6f869dbd9952be8a926e80c4f1e172845ab8d5f)
diff --git a/payload_consumer/download_action.h b/payload_consumer/download_action.h
index d0e6000..786db20 100644
--- a/payload_consumer/download_action.h
+++ b/payload_consumer/download_action.h
@@ -166,7 +166,8 @@
 
   // For reporting status to outsiders
   DownloadActionDelegate* delegate_;
-  uint64_t bytes_received_{0};
+  uint64_t bytes_received_{0};  // per file/range
+  uint64_t bytes_received_previous_payloads_{0};
   uint64_t bytes_total_{0};
   bool download_active_{false};