Cleaning up scrollbar logic to properly calculate stable extents.

- Removing old logic which assumed that views were the same size,
  especially now we can have variable dividers, etc.
- Simplifying old scroll position logic.
- Removing unnecessary prediction icon layout (same as normal icon)

Bug: 30023608
Change-Id: I39e1126fa586a76a9bdd3ff38cd6e360ac3021e6