Set-up dummy animation when setting a replacing window token

Set-up dummy animation so we can start treating windows associated
with the replacing app token like they are in transition before the
new app window is ready for us to run the real transition animation.
This allows us to make the right decisions at various call points
where an animation is expected to be running for a replacing window
but the real animation isn't set yet.

Also, removed unused field indicating if an app token is animating
a replacement window since it was always set to true and checked/set
to the same value as AppWindowToken.mWillReplaceWindow.

Bug: 25645069
Change-Id: Ie216ed5bd60fb2a15da61c7050c9344c48b1c5fb