Fix Trusted Filter to work with Mt. Wilson `vtime`

The Trusted Filter expects that `vtime` is returned from the
attestation server in an ISO8601 format, but the Mt. Wilson
attestation server can return it in a locale appropriate string format
instead. There's no way to configure Mt.  Wilson to do the right thing
so we should just try to parse the returned `vtime` as a string
formatted date.

Closes-Bug: #1353029

Change-Id: Ic7351e0463c014321bdb4fcfeba90ac51460b325
Merge "Fix Trusted Filter to work with Mt. Wilson `vtime`"