Set default max_thread to 256 am: 6fbf3bec2b am: fe1d5065c8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3314883

Change-Id: I0b971fa09f2327aa2a59d5d250bc3fcdd659905b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>