Improved timing of when system sends multi-window change event to app

We were previously sending multi-window/PiP mode changed events to
apps once the task config is changed in activity manager.
However, the actual changing of the multi-windowing or PiP mode
might not be fully complete when the app receives the event so they
might be wrong state information they query for like currnet config.
We now schedule the events to be send once the current transaction
cycle is done.

Bug: 26877409
Change-Id: I393b56035bb4197f99b3db3d27c0599835b5f86c