vold: allow vold to mount zram swap device

Address the below bootup denial needed for zram device.

[   43.388127] type=1400 audit(859.029:6): avc: denied { getattr } for
pid=1570 comm="e2fsck" path="/dev/block/zram0" dev="tmpfs" ino=15387
scontext=u:r:vold:s0 tcontext=u:object_r:swap_block_device:s0
tclass=blk_file permissive=0

CRs-Fixed: 931731
Change-Id: Icd308a464a89b9edc5cc6d3f88fee33997037452