Bug 1534207 Search Widget missing TELEMETRY probe for searches initiated r=petru

Added a telemetry probe (unique source value) to allow data science to measure percentage of searches initiated from the widget.

Differential Revision: https://phabricator.services.mozilla.com/D22956

--HG--
extra : moz-landing-system : lando

mobile/android/base/java/org/mozilla/gecko/BrowserApp.java

@@ -893,6 +893,7 @@ public class BrowserApp extends GeckoApp
         }
 
         MmaDelegate.track(INTERACT_WITH_SEARCH_WIDGET_URL_AREA);
+        Telemetry.sendUIEvent(TelemetryContract.Event.SEARCH, TelemetryContract.Method.WIDGET);
 
         switch (input) {
             case TEXT:

mobile/android/docs/uitelemetry.rst

@@ -198,6 +198,9 @@ Events
 ``search.setdefault.1``
   Sent when the user sets a search engine to be the default.
 
+``search.widget.1``
+  Sent when the user initiates a search through the widget.
+
 ``share.1``
   Sharing content.
 

mobile/android/geckoview/src/main/java/org/mozilla/gecko/TelemetryContract.java

@@ -88,6 +88,9 @@ public interface TelemetryContract {
         // Set default search engine.
         SEARCH_SET_DEFAULT("search.setdefault.1"),
 
+        // Searches initiated from the widget.
+        SEARCH_WIDGET("search.widget.1"),
+
         // Sharing content.
         SHARE("share.1"),