Prepare /data/user/0 and /data/media/obb during initUser0

Prepare these directories during initUser0.  This greatly shortens the
gap between the creation and encryption of /data/user/0, and this makes
it possible to remove init's write access to all directories containing
per-user encrypted directories.

Bug: 156305599
Change-Id: Ibf3d25356e8f0bca70da078c5d2428ae8615240e