Defer resizing invisible stacks while drag resizing

Works around a source of jank when drag resizing in split
screen mode: instead of immediately resizing the (potentially
numerous) invisible secondary stacks, we defer that until
the user lets go of the handle.

Change-Id: I3b9faa83005fa86185d4e51b2849e3a826b7f6a9
Fixes: 78214347
Test: Open a gazillion (resizeable) tasks. Enter split screen. Drag handle, verify there is no jank
Test: atest RectTest