[Material] Add focus, highlight, and keyboard shortcuts to Slider (#53945)

packages/flutter/lib/src/material/slider_theme.dart

@@ -505,7 +505,8 @@ class SliderThemeData with Diagnosticable {
   /// [Slider] is disabled.
   final Color disabledThumbColor;
 
-  /// The color of the overlay drawn around the slider thumb when it is pressed.
+  /// The color of the overlay drawn around the slider thumb when it is
+  /// pressed, focused, or hovered.
   ///
   /// This is typically a semi-transparent color.
   final Color overlayColor;

packages/flutter/test/painting/system_fonts_test.dart

@@ -194,11 +194,9 @@ void main() {
       SystemChannels.system.codec.encodeMessage(data),
         (ByteData data) { },
     );
-     final RenderObject renderObject = tester.renderObject(find.byType(Slider));
-
-     bool sliderBoxNeedsLayout;
-     renderObject.visitChildren((RenderObject child) {sliderBoxNeedsLayout = child.debugNeedsLayout;});
-     expect(sliderBoxNeedsLayout, isTrue);
+    // _RenderSlider is the last render object in the tree.
+    final RenderObject renderObject = tester.allRenderObjects.last;
+    expect(renderObject.debugNeedsLayout, isTrue);
   });
 
   testWidgets('TimePicker relayout upon system fonts changes', (WidgetTester tester) async {