split-view: speculative fix for crash

From the crash stack, it looks like the following events happened:
- User snaps a window from the window layout menu
- The window is snapped, Overview is opened on other side of the screen
- Overview tries to stop all ongoing animations for windows that are
  going to show in Overview first before initializing Overview grids
- Crash happened when accessing overview grid

The problem seems to be that the snapped window shouldn't be put in the Overview window list so when Overview starts, it might still go through its window bounds change animation which Overview then tried to stop. The snapped window should have excluded from the Overview window list. From ash::window_util::ShouldExcludeForOverview(), it seems to suggest that the window is not thought to be the default snapped window, which indicates the default_snap_position_ is not set correctly.

Bug: 420963933
Change-Id: Iaa4f460d65d03b4f4abb1ae8a9eef694b1da2b85
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6614381
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1469678}