Cleanup dangling mount on update_engine start am: 1df000ab2e am: 4e3962e108 am: 6a0d59cb81

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1978849

Change-Id: I347a39565cdd4a7154b2b63d7b64c9b86625c04c