Restrict SCORE_NETWORKS permission to signature|system.

As long as the APIs aren't ready to be un-hidden, there's no need to
allow non-system apps to access them. This can be reverted when the
APIs are finalized and we have mechanisms in place for selecting
between scorers installed on the device, rather than only a
preselected scorer bundled with the system.

Change-Id: I745b4c691eccfb526014d4cb71639db2119e4fdb