Fix `TabBarTheme.indicatorColor` not applied in Material 2 (#132123)

fixes [[Proposal] Improve TabBarTheme styling API for indicator color ](https://github.com/flutter/flutter/issues/130392)

### Description

This fixes an issue where the `TabBarTheme.indicator` isn't applied in Material 2 and also adds indicator color tests for both M3 and M2.

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

/// Flutter code sample for [TabBar].

void main() => runApp(const TabBarApp());

class TabBarApp extends StatelessWidget {
  const TabBarApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        useMaterial3: false,
        tabBarTheme: const TabBarTheme(
          indicatorColor: Colors.amber,
        )
      ),
      home: const TabBarExample(),
    );
  }
}

class TabBarExample extends StatelessWidget {
  const TabBarExample({super.key});

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      initialIndex: 1,
      length: 3,
      child: Scaffold(
        appBar: AppBar(
          title: const Text('TabBar Sample'),
          bottom: const TabBar(
            tabs: <Widget>[
              Tab(
                icon: Icon(Icons.cloud_outlined),
                text: 'Cloudy',
              ),
              Tab(
                icon: Icon(Icons.beach_access_sharp),
                text: 'Sunny',
              ),
              Tab(
                icon: Icon(Icons.brightness_5_sharp),
                text: 'Rainy',
              ),
            ],
          ),
        ),
        body: const TabBarView(
          children: <Widget>[
            Center(
              child: Text("It's cloudy here"),
            ),
            Center(
              child: Text("It's rainy here"),
            ),
            Center(
              child: Text("It's sunny here"),
            ),
          ],
        ),
      ),
    );
  }
}
``` 
	
</details>

### Before
![Screenshot 2023-08-08 at 15 39 24](https://github.com/flutter/flutter/assets/48603081/9030e025-8615-45d0-a337-87ba2fdf6ca3)

### After
![Screenshot 2023-08-08 at 15 39 07](https://github.com/flutter/flutter/assets/48603081/4b98ac55-2d51-4a4c-93ba-7d36dc4be1d8)

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

@@ -1220,10 +1220,7 @@ class _TabBarState extends State<TabBar> {
       return tabBarTheme.indicator!;
     }
 
-    Color color = widget.indicatorColor
-      ?? (theme.useMaterial3
-         ? tabBarTheme.indicatorColor ?? _defaults.indicatorColor!
-         : Theme.of(context).indicatorColor);
+    Color color = widget.indicatorColor ?? tabBarTheme.indicatorColor ?? _defaults.indicatorColor!;
     // ThemeData tries to avoid this by having indicatorColor avoid being the
     // primaryColor. However, it's possible that the tab bar is on a
     // Material that isn't the primaryColor. In that case, if the indicator

packages/flutter/test/material/tab_bar_theme_test.dart

@@ -1132,4 +1132,90 @@ void main() {
       expect(tabTwoRect.right, equals(tabTwoRight));
     });
   });
+
+  testWidgets('Material3 - TabBar indicator respects TabBarTheme.indicatorColor', (WidgetTester tester) async {
+    final List<Widget> tabs = List<Widget>.generate(4, (int index) {
+      return Tab(text: 'Tab $index');
+    });
+
+    final TabController controller = TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+    );
+
+    const Color tabBarThemeIndicatorColor = Color(0xffff0000);
+
+    Widget buildTabBar({ required ThemeData theme }) {
+      return MaterialApp(
+        theme: theme,
+        home: Material(
+          child: Container(
+            alignment: Alignment.topLeft,
+            child: TabBar(
+              controller: controller,
+              tabs: tabs,
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildTabBar(theme: ThemeData(useMaterial3: true)));
+
+    RenderBox tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    expect(tabBarBox,paints..rrect(color: ThemeData(useMaterial3: true).colorScheme.primary));
+
+    await tester.pumpWidget(buildTabBar(theme: ThemeData(
+      useMaterial3: true,
+      tabBarTheme: const TabBarTheme(indicatorColor: tabBarThemeIndicatorColor)
+    )));
+    await tester.pumpAndSettle();
+
+    tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    expect(tabBarBox,paints..rrect(color: tabBarThemeIndicatorColor));
+  });
+
+  testWidgets('Material2 - TabBar indicator respects TabBarTheme.indicatorColor', (WidgetTester tester) async {
+    final List<Widget> tabs = List<Widget>.generate(4, (int index) {
+      return Tab(text: 'Tab $index');
+    });
+
+    final TabController controller = TabController(
+      vsync: const TestVSync(),
+      length: tabs.length,
+    );
+
+    const Color themeIndicatorColor = Color(0xffff0000);
+    const Color tabBarThemeIndicatorColor = Color(0xffffff00);
+
+    Widget buildTabBar({ Color? themeIndicatorColor, Color? tabBarThemeIndicatorColor }) {
+      return MaterialApp(
+        theme: ThemeData(
+          indicatorColor: themeIndicatorColor,
+          tabBarTheme: TabBarTheme(indicatorColor: tabBarThemeIndicatorColor),
+          useMaterial3: false,
+        ),
+        home: Material(
+          child: Container(
+            alignment: Alignment.topLeft,
+            child: TabBar(
+              controller: controller,
+              tabs: tabs,
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildTabBar(themeIndicatorColor: themeIndicatorColor));
+
+    RenderBox tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    expect(tabBarBox,paints..line(color: themeIndicatorColor));
+
+    await tester.pumpWidget(buildTabBar(tabBarThemeIndicatorColor: tabBarThemeIndicatorColor));
+    await tester.pumpAndSettle();
+
+    tabBarBox = tester.firstRenderObject<RenderBox>(find.byType(TabBar));
+    expect(tabBarBox,paints..line(color: tabBarThemeIndicatorColor));
+  });
 }

packages/flutter/test/material/tabs_test.dart

@@ -488,7 +488,6 @@ void main() {
 
     const double indicatorWeight = 3.0;
 
-
     final RRect rrect = ui.ParagraphBuilder.shouldDisableRoundingHack
       ? RRect.fromLTRBAndCorners(
           64.75,