Extend run-as with optional --user argument.

1. Calculate AID for spawned process as (100000 * $user) + uid_from_packages.list
2. Use /data/user/$user/$packageDir as a root of a new process if $user != 0.

Change-Id: I761dfb481114bd51e5a950307fcaf403e96eef10
(cherry picked from commit da31778f)