[flexiglass] Update NSSL clipping based on Composable placeholder

NSSL's rounded clipping bounds are now updated based on the Composable placeholder's bounds, fixing the issue where NSSL was invisible due to the clipping bounds not being set in Flexiglass. The corner radius has also been moved from being hardcoded in the Composable into NotificationStackAppearanceRepository.

Bug: 296118689
Test: manually verified NSSL is visible with Flexiglass flags on.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Iaa16af6f17aa806c40cc955170c40841cf3b8d9e