Don't keep pregrants for non-system apps after partial uninstallation

Non-system apps may be marked as uninstalled but not fully removed if
they are still installed in another user on the device. However, since
they aren't system apps we should still prefer considering them gone and
not preserve their pregrants, maintaining the same behavior as U.

Fixes: 365920333
Test: presubmit
Change-Id: If11c0c74216a2daba557e5332124b549ff10c42d