Don't round the top corners of the topmost magnetic target

While the view at index 0 already has its top corner roundness set to 0, there may be cases (such as when a group header stops magnetic movement) where the first view is not at index 0, but still should not have changed top roundness.

Bug: 421614121
Test: manual, by
- creating enough notifications in notify to get autogrouped and bundled
- expanding bundle and group within the bundle
- drag on child notifications of the group within magnetic pull range, confirm corners do not round
Flag: EXEMPT bugfix

Change-Id: Idb0b5afb47892fffa71ab3721bb731164fbf8408