Ensure all events from a showing window are dispatched.

Accessibility services may opt-in to introspect the interactive
windows on the screen. If window introspection is enabled there
is a case where some events from a showing window are received
before the updated window state from the window manager. Now the
window manager sends over the windows before notifying the app
for the focus change.

bug:18625996

Change-Id: Ic481e01efbe12dc92f090f799feeb236672fc7b3