autocomplete: fix duplicate/missing tracking events

autocomplete: fix duplicate/missing tracking events

The submit-form and click-result handlers are in some cases being fired for the
same user interaction. The causes duplicate events to be collected from
WikimediaEvents. See also Ic1e0d08d for tests of this functionality along with
updates necessary to ensure all the right events are showing up on the backend.

Additionally adjusts to ensure all reports of position selected/clicked
are 0 indexed.

Change-Id: Ie7348e470754746c85f53dbe447b88176c2a0bcc