cow estimation: use old partition size

Old partition size needs to be used for xor writes. Otherwise offset
checks will fail resulting in a failed Write()

Bug: 332640740
Test: th
Change-Id: Ic8d5418dd0b3af652dd97521f3ea79d90c6ca706
diff --git a/aosp/cow_converter.cc b/aosp/cow_converter.cc
index 3e8e5fd..32aa12f 100644
--- a/aosp/cow_converter.cc
+++ b/aosp/cow_converter.cc
@@ -95,6 +95,7 @@
                                   manifest.block_size(),
                                   cow_writer.get(),
                                   partition.new_partition_info().size(),
+                                  partition.old_partition_info().size(),
                                   false));
   TEST_AND_RETURN_FALSE(cow_writer->Finalize());
   return true;