Fix crash in a11y cache

Caused by bad data from an app where a node was its own ancestor.
When the node changed, the copy in the cache was recycled while
flushing the descendants. Now recovering more gracefully.

Bug: 37674439
Test: Adding a unit test for this case, ran a11y cts and unit tests,
verified that the app reporting bad data no longer crashes TalkBack.
Change-Id: I10bc9df2b4821529334a19aa27719c1510187690