MDL-36943 Do not notify about already installed updates

Before this patch, the available_update_checker::cron_notifications()
method accessed the availableupdates property. But for plugins, that property
basically contains the most recent version available. So we were missing
the check against the actual version installed.

The fix was simple - obtain available updates via the available_updates()
method that performs the check.

lib/pluginlib.php

@@ -1195,15 +1195,18 @@ protected function cron_notifications(array $changes) {
                             // $componentupdate is a real update with higher version
                             $notifications[] = $componentupdate;
                         } else {
-                            // use the plugin_manager to check if the reported $componentchange
-                            // is a real update with higher version. such a real update must be
-                            // present in the 'availableupdates' property of one of the component's
-                            // available_update_info object
+                            // Use the plugin_manager to check if the detected $componentchange
+                            // is a real update with higher version. That is, the $componentchange
+                            // is present in the array of {@link available_update_info} objects
+                            // returned by the plugin's available_updates() method.
                             list($plugintype, $pluginname) = normalize_component($component);
-                            if (!empty($plugins[$plugintype][$pluginname]->availableupdates)) {
-                                foreach ($plugins[$plugintype][$pluginname]->availableupdates as $availableupdate) {
-                                    if ($availableupdate->version == $componentchange['version']) {
-                                        $notifications[] = $componentupdate;
+                            if (!empty($plugins[$plugintype][$pluginname])) {
+                                $availableupdates = $plugins[$plugintype][$pluginname]->available_updates();
+                                if (!empty($availableupdates)) {
+                                    foreach ($availableupdates as $availableupdate) {
+                                        if ($availableupdate->version == $componentchange['version']) {
+                                            $notifications[] = $componentupdate;
+                                        }
                                     }
                                 }
                             }