Move "max to height" roundness logic to Roundable.kt

The sum of top and bottom corner radii should be at max the clipped
height of the roundable view.

This logic was spread out between the ExpandableOutlineView and the
ActivateableNotificationView. This CL moves it to the Roundable.kt.

Bug: b/243302608
Test: post HUNs and observe the appear/disappear animations
Test: see the corners changing as Notifs scroll into the Shelf
Change-Id: I3a615e81f7bfbb9e9f512d0b3ce745c019ea637f