Tell vold to use the fstype defined in fstab when formatting

If, for whatever reason, a voldmanaged volume had a different filesystem
than the one defined in the fstab, then this filesystem was kept when
formatting. Change the logic to use the filesystem defined in the fstab when
available.

Change-Id: Iffbb5dc42838c1585c21f3ffa66b16921a16abd5