Set default max_thread to 256 am: 6fbf3bec2b

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

Change-Id: I3f94fc163ba06ee7ac237e35386fb2716c10139c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>