Reland "splitview: End split view if partial overview is not started"

This is a reland of commit a6f0b23b74bc60d7da7bcdade6bde32616b45db0

Original change's description:
> splitview: End split view if partial overview is not started
>
> This fixes the issue where we weren't clearing window observations if
> we weren't in clamshell split view, i.e. window + partial overview.
> Note we need `primary|secondary_window_` up until the point where
> partial overview is started, after which we need to end split view.
>
> Test: SnapGroupTest*, SplitViewControllerClamshellTest*
> Bug: b/333600706
> Change-Id: I35faaefd814f30df898d08406acd0255c4511945
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5454124
> Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
> Commit-Queue: Sophie Wen <sophiewen@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1288514}

Bug: b/333600706
Change-Id: Ia2ba4f516781bf4c3ea493ccd09d7c2384ff32de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5462440
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sophie Wen <sophiewen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1288797}