[fenix] For https://github.com/mozilla-mobile/fenix/issues/6558 - change from modifying event to event listener

The initial design replaced the onload method, which was used by others such as Bing.
With listeners it's additive, we do not replace anything.

app/src/main/assets/extensions/ads/ads.js

@@ -25,18 +25,26 @@ function sendLinks() {
     browser.runtime.sendNativeMessage("MozacBrowserAds", message);
 }
 
+const events = ["pageshow", "load", "unload"];
 var timeout;
 
-window.onload = function() {
+const eventLogger = event => {
+  switch (event.type) {
+    case "load":
         timeout = setTimeout(sendLinks, ADLINK_CHECK_TIMEOUT_MS);
-};
-
-window.onpageshow = function(event) {
+        break;
+    case "pageshow":
         if (event.persisted) {
           timeout = setTimeout(sendLinks, ADLINK_CHECK_TIMEOUT_MS);
         }
-};
-
-window.onunload = function() {
+        break;
+    case "unload":
         clearTimeout(timeout);
+    default:
+        console.log('Event:', event.type);
+  }
 };
+
+events.forEach(eventName =>
+  window.addEventListener(eventName, eventLogger)
+);