Rename stdout and stderr to stdout_str and stderr_str am: d76a8ac551 am: d2c06faf65 am: b442f5b8ae

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1931288

Change-Id: Iff230c8dc6a9fb31f1e9ed9fd918c989a0bf63b1