Enable vold to set level from user.

Enable vold to set level from user.

We want various per-user directories to have their SELinux MLS level
set to restrict access from other users, as an improvement to user
isolation.

We extend vold_prepare_subdirs to implement this if a flag is
set. vold itself then sets the flag based on a new property,
ro.vold.level_from_user. This is to allow testing of further
incremental work to ensure system apps correctly handle the new
restriction on different devices rather than causing immediate
breakage. Eventually this will go away and the restriction will apply
everywhere.

Bug: 141677108
Test: Manual, with and without propery set.
Change-Id: I8e2207bd94b487bdcc09fd4d80b031027dfea1e3