Write packages.list when adding/removing users.

FUSE daemons now rely on getting per-user GID information when
packages.list is written.  Normal secondary user adding/removing
usually has enough PackageManager traffic to trigger a side-effect
rewrite, but this change writes explicitly to handle guest users.

Also obtain the user list once, and exclude dying users.  During
user creation we manually splice in the user ID that we're bringing
online.

Bug: 19924661
Change-Id: Icc5b1b169300c9dc12099be12651acbf89d6bea9