Make the focus node on SelectableRegion optional. (#158994)

## Description

This makes the `focusNode`  for `SelectableRegion` optional so that:
- Users of the widget are no longer required to use `SelectableRegion`
from within a `StatefulWidget`
- They aren't likely to forget to dispose of a node they didn't supply.
- Simpler to use, and the node is not used very often anyhow.

Also made the `SelectableRegion` sample actually use `SelectableRegion`.

## Tests
- Modified all the `SelectableRegion` tests to remove 3 identical lines
of boilerplate from each (except 2, which actually used their focus
nodes).

examples/api/lib/material/selectable_region/selectable_region.0.dart

@@ -15,7 +15,8 @@ class SelectableRegionExampleApp extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-      home: SelectionArea(
+      home: SelectableRegion(
+        selectionControls: materialTextSelectionControls,
         child: Scaffold(
           appBar: AppBar(title: const Text('SelectableRegion Sample')),
           body: const Center(