Add a sysprop to delay merge by fix duration am: 8bcb221f3e am: add797b37f

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2834791

Change-Id: I9861314ce9d7e621bfa400081a3b65bee0a14c07
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>