Different blkid and fsck execution domains.

vold works with two broad classes of block devices: untrusted devices
that come in from the wild, and trusted devices.

When running blkid and fsck, we pick which SELinux execution domain
to use based on which class the device belongs to.

Bug: 19993667
Change-Id: I44f5bac5dd94f0f76f3e4ef50ddbde5a32bd17a5