Don't clear a11y focus with lock held

The bug I'm fixing is kind of weird, but in the trace it
showed lock contention with onWindowsForAccessibilityChanged
holding the lock for a long time. So I've cleaned up that
method a bit, looking for trouble. The only concrete thing
I see that's wrong is that I was clearing a11y focus, which
involves calling the app that last reported having focus.
That was done with the lock held. Now doing it through the
handler.

Bug: 77298850
Test: Ran a11y CTS and unit tests.
Change-Id: I667707412a0e3ba98639d0cb43858e8c6b52a5a7