manifest: Track specific revision of svox to fix security vulnerability

Unfortunately, github does not allow us to fork svox, so we'll use
the particular commit ID which fixes CVE-2017-13285 RCE vulnerability.
Even though this revision originates from Oreo, the difference to the
current nougat branch is negligible.

Change-Id: Ib0e6e7c7be8ff60584e1c6cb5fb677d87e1dbc12