Changed delta_generator to make use of --max_threads am: cbe8e633f2

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2562931

Change-Id: I65634f50cbb8b402fd4633fd82da9e49eab0f52e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>