Changed delta_generator to make use of --max_threads am: cbe8e633f2 am: 607027dea6

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

Change-Id: Id9b68d423e9cbb28f39280db5110658bc36fa810
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>