Enable run-as to read packages.list now owned by package_info.

The group ownership of the package database
/data/system/packages.list read by run-as was changed in
977a9f3b from "system" to
"package_info". run-as currently changes its effective group to
"system" and is thus unable to read the database.

This CL fixes the issue by making run-as change its effective group
to "package_info" for reading the package database.

Bug: 10411916
Change-Id: Id23059bfb5b43264824917873a31c287f057ce4e