vold: Fix fsck on public volumes

vold: Fix fsck on public volumes

* Fsck was hitting a neverallow on public volumes not formatted in vfat
  because it was always using the trusted context
* Always run trusted fsck for private volumes and untrusted for public
* Exfat/ntfs are always untrusted, because they are not supported for
  private volumes, like vfat

Change-Id: I0a6ee9aea907bae9ed097b920df0559df7b45d7d