Change split drag zones to split snap.

Changes the drag transition on the left/right sides of the screen from
changing into splitscreen windowing to remaining in desktop with split
snap bounds.

Also refactors VeiledResizeTaskPositioner to do two things:

1: Consolidate mVeilIsVisible and mIsResizingOrAnimatingResize as these
   were being used identically.
2: Check resizing status before calling hideResizeVeil; hide being
   called on drag end would prevent the veil from showing during the
   split snap animation.

Bug: 280828642
Bug: 317286772
Test: Manual
Change-Id: I61b8da0d2abcf698d8d0ff4c9b5ec7bcce256bbf