Add --max_threads to OTA generation

Adding an option to configure max number of threads used during
ota_from_target_files

Bug: 277794445
Test: tested ota_from_target_files
(cherry picked from https://android-review.googlesource.com/q/commit:a30989bafa820faf24eca7e11e48f8f122d7991f)
Merged-In: I9bae92ee43c21926434ab04563a2b4f0678b73eb
Change-Id: I9bae92ee43c21926434ab04563a2b4f0678b73eb
this patch is necessary for partner to improve OTA generation b/277794445