Set correct SELinux labels on new user directories

Make vold explicitly set the appropriate fscreate SELinux context when
creating per-user subdirectories such as /data/user/$userId.  This is
needed for these subdirectories to get the correct SELinux labels after
the sepolicy change https://r.android.com/2078213 changes their parent
directories to have different labels.

Note: the helper function being changed is also used for some other
directories, such as subdirectories of /data/misc/vold.  But this is
fine since they still get the same labels as before.

Test: see https://r.android.com/2078213
Bug: 156305599
Change-Id: Id61c2d985144007059c563cec91b1355176e915c