Add a sysprop to delay merge by fix duration am: 8bcb221f3e am: 76e1ab6045

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

Change-Id: I0b05a28524f65a533a41b311e11c19bbd47eebd7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>