actionable.py: Do not print next task banner in handler callback (#15698)

Fix actionable callback plugin to not print the banner of the previous
task.

When a handler is executed there is no task banner, so in case it is run,
it will reference the banner from the preceding task.

**Author:** @hvhaugwitz

Test case:

      ---

      - name: actionable filter
        hosts: all
        handlers:
          - name: handler
            command: "true"
        tasks:
          - name: task 1
            file: path=/tmp/test state=touch
            notify: handler
          - name: task 2
            file: path=/tmp/test state=absent
          - name: task 3
            file: path=/tmp/test state=absent
          - name: task 4
            file: path=/tmp/test state=absent
          - name: task 5
            file: path=/tmp/test state=absent
          - name: task 6
            file: path=/tmp/test state=absent

Example output:

BEFORE
------

      PLAY [actionable filter] *******************************************************

      TASK [task 1] ******************************************************************
      changed: [localhost]

      TASK [task 2] ******************************************************************
      changed: [localhost]

      RUNNING HANDLER [handler] ******************************************************

      TASK [task 6] ******************************************************************
      changed: [localhost]

      PLAY RECAP *********************************************************************
      localhost                  : ok=8    changed=3    unreachable=0    failed=0

AFTER
-----

      PLAY [actionable filter] *******************************************************

      TASK [task 1] ******************************************************************
      changed: [localhost]

      TASK [task 2] ******************************************************************
      changed: [localhost]

      RUNNING HANDLER [handler] ******************************************************
      changed: [localhost]

      PLAY RECAP *********************************************************************
      localhost                  : ok=8    changed=3    unreachable=0    failed=0

lib/ansible/plugins/callback/actionable.py

@@ -33,6 +33,10 @@ class CallbackModule(CallbackModule_default):
         self.last_task = None
         self.shown_title = False
 
+    def v2_playbook_on_handler_task_start(self, task):
+        self.super_ref.v2_playbook_on_handler_task_start(task)
+        self.shown_title = True
+
     def v2_playbook_on_task_start(self, task, is_conditional):
         self.last_task = task
         self.shown_title = False