vold: do not try and unmount asec when unmounting non-primary storage

patch was lost in the merge

Change-Id: I9de6c2dc99c1484ac86fec7ee94eea5977a4aa11
vold: use blkid to detect filesystem type

In preparation for handling ext4 partitions via vold and fuse.

Change-Id: Ibaccded63a7a293ad53f16afb80448ddd493554c

Conflicts:
	Android.mk