Make sure we initialize to enable the default voice interaction service.

We weren't taking care of the case of an upgrade to L.  The main
change here is that the voice interaction service setting now has
an empty string when the user has explicitly said they don't want
one, so when it is null we will initialize it to its default.

Change-Id: Icdd30b4f09498f8928cea759a64628bd43bc5d0e