Let vold mount OBB files on external storage.

Fixes this specific violation:

type=1400 audit(1384468728.202:16): avc:  denied { read write } for
pid=271 comm="vold" name="test1.obb" dev="fuse" ino=3100664872
scontext=u:r:vold:s0 tcontext=u:object_r:sdcard_internal:s0
tclass=file

Bug: 11693888
Change-Id: I45d30ecabdf0bc8871f3dd67b5695ac909109d9a