Make update_engine_stable_client product_specific. am: 309c8a883e am: 6e641d743b am: abf732f0f4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1443150

Change-Id: I0b66107999730319e12f5634c8cbe155adfc2b0b