Defer copy splash screen to client after transaction committed.

For cold launch, usually there is an opening transition. And if the
app has launch another activity, there will do transfer splash screen
to the latest visible activity.
But the transaction of transfer starting window will happen after
first transition, so if core copy the splash screen to client before
the transfer starting window transaction apply, the latest top activity
would still hidden then nothing can be see on the screen.
So like remove starting window, we should also defer the copy action
after transfer starting window transaction done.

Bug: 295458327
Test: launch test app several times, to verify no flickering anymore.
Change-Id: I733fc19bc09cddf8beee01a85283197c0576964a