vold.te: allow BLKSECDISCARD

vold needs to securely delete content from various block devices. Allow
it.

Addresses the following denials:

type=1400 audit(0.0:66): avc: denied { ioctl } for comm="secdiscard" path="/dev/block/dm-3" dev="tmpfs" ino=17945 ioctlcmd=0x127d scontext=u:r:vold:s0 tcontext=u:object_r:dm_device:s0 tclass=blk_file permissive=0
type=1400 audit(0.0:43): avc: denied { ioctl } for comm="secdiscard" path="/dev/block/sda45" dev="tmpfs" ino=17485 ioctlcmd=127d scontext=u:r:vold:s0 tcontext=u:object_r:userdata_block_device:s0 tclass=blk_file permissive=0

Test: policy compiles.
Change-Id: Ie7b4b8ac4698d9002a4e8d142d4e463f8d42899a