Update screenshot shelf actions layout

- Nest the actions in the actions background.
- Round all the corners.
- Fork layouts/drawables where needed to avoid impacting unflagged
  behavior.
- Use a transparent divider between actions.
- Clip to the rounded outline to make the scrollview not clip in a
  rectangle.

Bug: 329659738
Test: Manual testing with adding/removing actions, scrolling
Flag: ACONFIG com.android.systemui.screenshot_shelf_ui DEVELOPMENT

Change-Id: I331f8ff689d90ae40cd0e9820ee34382df34a6ac
Merged-In: I331f8ff689d90ae40cd0e9820ee34382df34a6ac