Don't generate irrelevant a11y events - framework

Now before we fire an a11y event we check if this event has an
observer. As a bonus we don't push the state to clients if the
dynamic service config did not change.

Test: cts-tradefed run cts-dev -m CtsAccessibilityServiceTestCases

bug:69427760

Change-Id: Ie208e13b8557bb7a120198a43efcb79c2752f5db