[SharedUserSetting] fix shared user migration

In ag/16740473 we missed the case when a package is migrating off from
shared user. Thanks topjohnwu@ for pointing it out!

BUG: 219376382
Test: atest android.uidmigration.cts.SharedUserMigrationTest
Change-Id: I11d117a1ea61e63f18891eb002a7c475f3abbe2b