[pm] preserve installed state only for system app downgrade

This CL is currently a no-op because the installed state is already set to
true during the uninstallation for single-user scenarios. A follow-up CL
will actually change the installed state for single-user DELETE_KEEP_DATA.

Also adds a small refactoring such that all app data directory I/O called
from RemovePackageHelper.

BUG: 288142708
Test: presubmit
Change-Id: I286581355aa2139b3e7de8feed51eb13c528f42b