recreate a11y focused virtual node when findFocus

Added documentation that ViewRootImpl#mAccessibilityFocusedVirtualView
may be stale except for bounds and sourceNodeId. And recreate
the node when a11y services calls findFocus.

Checked other usages of mAccessibilityFocusedVirtualView(including
getter method) and verified they are OK.

Fix: b/192706021

Test: tested with Jetpack Compose progress indicator and radio button.
Without Jetpack Compose side workaround, this fix works well with
talkback google3 head.

Change-Id: I40c42011512e3d1fccd1ae8a16a62cbf500cf2ad