[Widget Preview] Isolate PageStorage scope in widget preview group expansion tile (#191378)

## Description

When a preview group is collapsed and re-expanded in the Widget
Previewer, descendant scrollables (which search ancestor elements for a
`PageStorageKey` when restoring scroll offset) inherit the
`PageStorageKey` from the `ExpansionTile`. They then attempt to read the
`ExpansionTile`'s boolean expansion state as a double, throwing a
`TypeError: true: type 'bool' is not a subtype of type 'double?'`.

This PR wraps the `ExpansionTile` children in a `PageStorage` widget
with a new `PageStorageBucket`, preventing descendant scrollable widgets
from inheriting the `ExpansionTile`'s `PageStorageKey`.

## Related Issues
Fixes https://github.com/flutter/flutter/issues/191242

## Tests
- Tested in widget preview scaffold template.