Don't send Accessibility events during NotificationContentView changes

Don't send Accessibility events from the NotificationContentView while
it is animating a layout change. Notify the showing ContentView at the end
of the animation, that its accessibility state might have changed.

Bug: 296578272
Test: atest NotificationContentViewTest
Test: run notification jank  microbenchmark test suites with abdt
Flag: NONE
Change-Id: I3721fb20769487f6d3581df15a890abb9ec59161