[pm] fix SCAN_AS_STOPPED_SYSTEM_APP for restoreDisabledSystemPackage

Use ParsedPackage to directly check if the package has a launcher entry
or not. This way, we can unify the system-apps-stopped code path for
1) installing new system apps after an OTA, and 2) restoring disabled system
apps after uninstalling the update.

BUG: 286459841
Test: manual with a system app that doesn't have a launcher entry
Test: also manually checked that a system app with a launcher entry will
be stopped by default

Change-Id: Ic51a9ceb7db6d887cebc390bb2878db8810cddbf