Move A11y events throttling away from View(RootImpl)

..and also extract common code into a common superclass

This also preserves the order of the throttled events
(TYPE_VIEW_SCROLLED & TYPE_WINDOW_CONTENT_CHANGED) with regards to the rest
of events by flushing any pending throttled events immediately if another
event is requested to be sent.

Test: ensure no new a11y CTS failures
Change-Id: I948a16716521974393aaa1cf822d0a0324e9ce3a