[Shell Transition] Defer remove splash screen window when it is

inTransition.

If splash screen window was in transition, the client side is unable
to draw because of Session#cancelDraw, which can also blocking the
reveal animation. But since the client side may alreday report draw
finish after prepareSync, defer the remove starting window signal
after the start transaction applied.

Bug: 246518648
Test: repeat run atest testConvertTranslucentOnTranslucentActivity
Change-Id: Ia6a555b5657f531dfed1d2f97e23302f7cd584a2