Adding @hide to UpdateEngineStable am: 4914ffd6b2

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

Change-Id: I803228bc74362b03612602be94a1d92066bf28ab
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>