pm: Verify shared-user priv-app install location

Apps that share a UID with a privileged app are privleged and
should live in /system/priv-app. Otherwise, fail
assertPackageIsValid().

On Taimen, this results in two additional apps failing:
com.android.providers.userdictionary
com.android.providers.downloads.ui

Test: Boot Taimen, verify apps are scanned correctly.
Bug: 71593002
Change-Id: I29b4dc8a2fea18248fe1f6aeee87ae3798028c60