snap-groups: Implement auto-snap for clamshell

In Snap Groups clamshell mode, windows will be auto-snapped, i.e. from
shelf, when split view overview session is active. This moves the
creation of AutoSnapController to SplitViewController for tablet mode, and SplitViewOverviewSession for clamshell mode.

This is needed so that we can move `StartOverview()` on window snapped
to SnapGroupController.

Note that AutoSnapController will snap via `WindowState::OnWMEvent()`
and not `SplitViewController::SnapWindow()` to support multiple snap groups.

Fixed: b/298265541
Test: SnapGroupEntryPointArm1Test.AutoSnapOneWindow
Change-Id: Ied26a1f474949b3f1e5ec9dba846f200d0185f50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4895207
Reviewed-by: Michele Fan <michelefan@chromium.org>
Commit-Queue: Sophie Wen <sophiewen@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1203029}