Adding @hide to UpdateEngineStable am: 4914ffd6b2 am: 8ccfd25204

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2893923

Change-Id: I5fac9d7d1dd8cd0a5d810f730e022c253f6e1718
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>