Collect change before starting transition

With shell transition enabled, the previous implemented freeform and
fullscreen change transition doesn't actually work. It is necessary to
start collecting, making the server side change, before actually request
starting the transition. Otherwise, the change cannot be calculated
correctly with the necessary information.

Bug: 279803578
Test: FreeformWindowingModeTests
Change-Id: I9c25a6c30e1f5d39288d8ece4f0ca4982551d26a