[piav2] Simplify UIDs in PacakgeInstaller app

Multiple UIDs were used in Pia, without distinctive use of each of them.
We must ensure each UID has a clear use in Pia.

This was implemented for Pia V1 in ag/26478352

Test: Manual. Install an APK and ensure app is installed
Bug: 26666746
Flag: android.content.pm.use_pia_v2
Change-Id: I9267817b97d8d004cce4b9604b6018c6904adbb6