MDL-69262 core: Make instance count variable as a static class variable

With the static function variable $instance, calls to
\action_menu_link::export_for_template() from its subclasses are stored
in different variables. This causes duplicate IDs when different
implementations of action menu links are rendered on the action menu
trigger/link template. (e.g. action_menu_link and
action_menu_link_secondary both rendered on the same page).
To make the incrementing uniform for the action_menu_link class and its
implementations, the $instance variable is moved out of the
export_to_template() method and is now made to a static class variable.

lib/outputcomponents.php

@@ -4682,6 +4682,12 @@ class action_menu_link extends action_link implements renderable {
      */
     public $actionmenu = null;
 
+    /**
+     * The number of instances of this action menu link (and its subclasses).
+     * @var int
+     */
+    protected static $instance = 1;
+
     /**
      * Constructs the object.
      *
@@ -4705,10 +4711,8 @@ class action_menu_link extends action_link implements renderable {
      * @return stdClass
      */
     public function export_for_template(renderer_base $output) {
-        static $instance = 1;
-
         $data = parent::export_for_template($output);
-        $data->instance = $instance++;
+        $data->instance = self::$instance++;
 
         // Ignore what the parent did with the attributes, except for ID and class.
         $data->attributes = [];