Add a sysprop to delay merge by fix duration am: 8bcb221f3e am: add797b37f am: c4ff02f70d

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

Change-Id: Ibc21599dbac24702e103cfaee0e72d2b8e977c8a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>