untrusted_apps: allow untrusted_apps to execute from /vendor/app

The typical use case is where vendor apps which run as untrusted apps
use libraries that are packaged withing the apk

Bug: 37753883
Test: Tested by runnig pre-installed app that packages a library from
      /vendor/app

Change-Id: I445144e37e49e531f4f43b13f34d6f2e78d7a3cf
Signed-off-by: Sandeep Patil <sspatil@google.com>