Increase enforced widget corner radius on hub to 28dp

Most first party widgets use a 28dp radius, meaning the previous radius
of 16dp lead to an inconsistent look on the hub.

Bug: 329515555
Fixed: 329515555
Test: verified appearance on device
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: Ica91745e4dc652ab10c72e852b3745e5e014a871