Bug 1057613 - Telemetry for search widget buttons

Added telemetry probes for pressing buttons for launching the browser and search on the Search Widget. https://bugzilla.mozilla.org/show_bug.cgi?id=1057613
I think this should also be a LAUNCH event, since it's launching the search activity. We usually reserve the SEARCH event for the user actually performing a search.

I also think we can remove the "launch-" prefix from the extras, since they will all be associated with LAUNCH events.