Clear completion marker upon resetting slot switch am: e76cf564f1

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2292522

Change-Id: If4faf7d51f22aa3151bc2339739c9a5afa7a2492
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>