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Disables SplitView when KioskNextShell is enabled.

Also disables snapping and the drag/snap indicators when
split view is disabled.

Bug: 966889
Test: run ash_unittests; manually by dragging windows in Overview Mode
      in a KioskNext session

Change-Id: Idb7a6c57ac89144af726ca3480fba5eaae12c3a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636907
Auto-Submit: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Stefan Kuhne <skuhne@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666119}