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

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

Change-Id: Idb9fabad25147e85e76afff069ed6dad68608181
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>