Don't stop animation when starting windows app token changes

If another activity is starting while we are still animating
the starting window of the previous activity to start, we
transfer the starting window of the old activity to the one that
is currently starting so we don't have to create another starting
window and also to reduce jank from the starting window animation
appearing to restart.

However, there were several conditions that led to the starting
window animation stopping when the transfers app tokens

1. Starting window animator not been removed from the previous
app token animator causing it to finish/remove the starting window
prematurily.
2. Starting window animator not been properly added to the new
app token animator causing the animation not be be picked up.
3. WMS.mSkipAppTransitionAnimation been set to false regardless of if
an app transition was actually prepared in WMS.prepareAppTransition()
4. WMS.mSkipAppTransitionAnimation not been set to true in all cases
where the starting window transfers tokens even though we don't want the
new app to do any transition animation is the starting window is
animating.
5. New app not setting its animation to dummy animation when the next
transition should be skipped due to starting window still animating.
6. Starting window animation been cleared for the new app in
WMS.handleAppTransitionReadyLocked() even for cases where we transferred
the animation from the previous app.

Also, cleaned up some code.

Bug: 20953232
Change-Id: I714d6bdfcdaeeaac29f9d464bab9f3e9c192e937