This prepares us for a CustomScrollView that takes slivers.

The only remaining client of ScrollableGrid (the old version) is Pesto,
which needs AppBar integration.

This patch removes unnecessary uses of scrollableKey from tests.
Scrollable2 will likely use a different mechanism for identifying
itself, so we want to focus on the uses of scrollableKey that will need
to be addressed in the new implementation.

Fixes #6591

It wasn't taking padding into account when deciding which children were
visible. I modeled the solution off of the way ScrollableList handles
this.

Fixes https://github.com/flutter/flutter/issues/5522