Improve handling of undecrypted encrypted /data

- Until the encrypted partition is decrypted, treat it as a disk image
  instead of a filesystem so that it is properly backed up using dd.
- Make Partition::Wipe() use the filesystem from fstab for disk images
  so that wiping an undecrypted /data makes it a plain unencrypted
  partition again.

Change-Id: I78db977d6348cfa66b0cfbc492fb5776f84db48d