snap-group: Disallow special `OverviewItemFillMode` for items in group

Demo: http://b/341750824#comment3
Fixed: b/341750824
Test: Added unit tests + manually
Change-Id: I25a70111e442d06c124771ae363fd3f01b105c9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5610532


Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Michele Fan <michelefan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312903}