vold: allow non-vfat devices to be used as ASEC

CM supports alternative file systems (which some devices specify
in the fstab as 'auto').

Change-Id: I7e78fe87a7efc49d6749d5892107085d55156cce
Merge pull request #3 from SlimRoms/kk4.4

vold: allow non-vfat devices to be used as ASEC