sepolicy[6/8]: allow vold to create files on external sdcard

This is required for ASEC support. Vold can already create and
access directories, but do not yet have the permission for files.

Signed-off-by: Humberto Borba <humberos@gmail.com>
Change-Id: I5082bbff692e5dc53c7000e4b3a293e42d33f901