Define stable AIDL interface for update_engine am: 2b291f0576 am: e7e72fae64 am: 08c7c5b717

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1371069

Change-Id: I4938b53119a2b105e9ecd439e2cfa9d10434fdd1