Fix flicker and other weirdness

- When moving tasks to/from fullscreen <-> docked stack, don't
replace windows, but preserve them.
- Fix a bug where we didn't called prepareFreezingBounds which
caused the clip rect to be set to (0, 0, 0, 0) for one frame

Bug: 26070457
Change-Id: I439c9b6612367babfbf31899022209eeee3e1be1