The "system service" you talk about is still part of Google Play Store package. What I'm talking about is an API on PackageManager level (part of OS/AOSP) and n

The "system service" you talk about is still part of Google Play Store package. What I'm talking about is an API on PackageManager level (part of OS/AOSP) and not inside Play Store. The whole mechanism you describe is unrelated to the issue of installing user apps without confirmation for alternative stores.