Fix a11y cache correctness bug

If two views change, their common ancestor can send
a subtree_changed event. But the cache is dropping
events that come from views that it hasn't cached.

Now clearing the cache entirely when a node not in
the cache sends a subtree changed event, since we
don't know which nodes are descendents and which
may have changed.

I'm not thrilled with this change, since it may
degrade performance, but I want to fix the correctness
problem quickly.

Bug: 111554539
Test: atest AccessibilityCacheTest
Change-Id: Ib32d3622cddd7001663943eff71e823d21f5e500