allow kernel to use vold file descriptors

Vold opens ASEC containsers on the sdcard, or OBB files from app's
home directories, both of which are supplied by vold. We need to
allow kernel threads to access those file descriptors.

Addresses the following denial:

  loop0   : type=1400 audit(0.0:28): avc: denied { use } for path="/mnt/secure/asec/smdl1159865753.tmp.asec" dev="mmcblk1" ino=19 scontext=u:r:kernel:s0 tcontext=u:r:vold:s0 tclass=fd permissive=0

Bug: 19516891
Change-Id: I5a3607b48f5e0e504e4b3fcaec19152c3784f49d