This fixes problems for Orca caused by object destruction when grabbing focus or setting the caret.
This patch also Fixes two bugs in the RemoteAccessible reuse code.
First, moved LocalAccessibles were previously being tracked even in the parent process if the cache was enabled.
While that was harmless, it was also unnecessary, since we only need to do that in order to send IPC notifications.
Second, when an OuterDocAccessible was moved, we were previously trying to shut down its child document, causing a crash.
Child documents must be left alone, since they are added ande removed differently to normal children.
Differential Revision: https://phabricator.services.mozilla.com/D135422
e3e9a67ed9