Avoid sending content changed a11y events from wrong views

The root cause of both attached bugs was the tree-merging algorithm in
ViewRootImpl.SendWindowContentChangedAccessibilityEvent converging on a
common predecessor that is marked not View#isImportantForAccessibility.

As a result, such unlucky content changed events were discarded.

Fixes: 72378611, 72950579
Test: ensure attached bugs are fixed
Change-Id: I3c3c66151b6cd4773de4eadd417709e9a61a7cf2