Detect system_ext provenance for APKs-in-APEX

The previous code mistakenly attributes APKs contained in an APEX loaded
to the system_ext partition as coming from the system partition. This
affects, for example, where PackageManager expects the permission
allowlist to be read from (e.g. /system_ext/etc/permissions/*.xml).

Test: atest ApkInApexTests (in
I4cec3fdf05f2ab1a3b5032870a74562aed53cb95)

Merged-In: I690618727ebf9a61f07b3925b30e7631d33c4e8e
Change-Id: I690618727ebf9a61f07b3925b30e7631d33c4e8e