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This vold command returns 0 if the given password matches the password
used to decrypt the device on boot.  It returns 1 if they don't match,
and it returns -1 on an internal error, and -2 if the device is not encrypted.

Also check the uid of the sender of the command and only allow the root and
system users to issue cryptfs commands.

Change-Id: I5e5ae3b72a2d7814ae68c2d49aa9deb90fb1dac5