Clear completion marker upon resetting slot switch am: e76cf564f1 am: 642a6f4a8b

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

Change-Id: Id69f7ef5e93b4121fab46a351f69602f4957e9d2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>