Consider overlay in system_ext as system's

When there is overlay package in system_ext, turn on POLICY_SYSTEM_PARTITION.
In other words, overlay pacakge in /system_ext is considered as system's one

Test: mv vendor/overlay/framework-res__auto_generated_rro.apk system_ext/overlay
      and then check if it works properly.
Bug: 136715327

Change-Id: Ib225368eae41203a8630f4310d26e9cf1afa706a