fs_mgr: autodetect filesystem type

* Multiple fstab lines (supported in android) cause recovery to fail
  to mount partitions if the fs type is not the same as the first
  fstab entry. So when we attempt to find an fstab entry, check it
  against blkid's determination of what filesystem type it is.

* If blkid fails, or if no entry matches the detected fs type,
  return the first found.

[yodevil: revamped for Q]
Change-Id: I7b720b534185fc7cb3860833aa49f748e02e3f6c