Dont dispatch a11y events that have no subscribers

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