Only wait for merge delay if there's merge pending am: a4650bd30e

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2886530

Change-Id: I8f2383d16e1ee3c79fd54fa1d8b9374ebd1bd8c4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>