Move creation of /data/user/0 and /data/media/obb to vold

To prevent bugs, directory creation and encryption should happen
together.  /data/user/0 (and its "alias" /data/data) is a per-user
encrypted directory; such directories can only be encrypted by vold.
Therefore, move its creation to vold as well.

Besides closing the uncomfortably-large gap between the creation and
encryption of /data/user/0, this allows removing init's write access to
/data/user and similar directories (SELinux type system_userdir_file) to
prevent any such issues from being reintroduced in the future.

To also allow removing init's write access to /data/media (SELinux type
media_userdir_file), which also contains per-user encrypted directories,
also move the creation and encryption of /data/media/obb to vold.

Bug: 156305599
BYPASS_INCLUSIVE_LANGUAGE_REASON=Linux API ("slave" mount flag)
Change-Id: I7245251eeb56b345b6c7711482c0aa5848648edb