Require vendor users and groups to start with vendor_

Require that users and groups found in /vendor/etc/{passwd,group}
start with vendor_.  This is needed to compliance with Treble as
without this prefix, it is possible for a new system image to create a
user/group name that a vendor has already used, causing a collision.

Bug: 79528966
Test: new unit test
Change-Id: I07500641e165f41526a8101592d83fa174e7a711