This allows to avoid A11yManager -> A11yManagerService IPC, when there's no
subscribers to a given event
Test: steps:
- Enable A11yManager.DEBUG
- Navigate through a few random activities
- In logcat, ensure log messages are present, notifying that certain events
won't be dispatched
Change-Id: Ia019fb66053f10095b3651407d09de8e89cdd227