Fix floating SnackBar is not centered when RTL and Material 2 (#147861)

## Description

This PR fixes floating `SnackBar` positioning when the text direction is RTL and the theme uses Material 2.

In https://github.com/flutter/flutter/pull/140215, I fixed the `SnackBar` position for M3/RTL, but while doing so I broke the positioning for M2/RTL... Unfortunately, there was no existing test for this case.
The solution is to not rely on `TextDirection` to compute the `SnackBar` position: the `SnackBar` is centered in both cases so the text direction has no impact on its positioning (it had some impact in the `SnackBar` content, but this is managed correctly).

## Related Issue

Fixes https://github.com/flutter/flutter/issues/147838.

## Tests

Adds 1 test.

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

@@ -1217,13 +1217,7 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
           : contentBottom;
       }
 
-      double xOffset = 0.0;
+      final double xOffset = hasCustomWidth ? (size.width - snackBarWidth!) / 2 : 0.0;
-      if (hasCustomWidth) {
-        xOffset = switch (textDirection) {
-          TextDirection.rtl => (snackBarWidth! - size.width) / 2,
-          TextDirection.ltr => (size.width - snackBarWidth!) / 2,
-        };
-      }
       positionChild(_ScaffoldSlot.snackBar, Offset(xOffset, snackBarYOffsetBase - snackBarSize.height));
 
       assert((){

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

@@ -819,7 +819,7 @@ class _SnackBarState extends State<SnackBar> {
           animation: heightM3Animation,
           builder: (BuildContext context, Widget? child) {
             return Align(
-              alignment: AlignmentDirectional.bottomStart,
+              alignment: Alignment.bottomLeft,
               heightFactor: heightM3Animation.value,
               child: child,
             );

packages/flutter/test/material/snack_bar_test.dart

@@ -2601,7 +2601,7 @@ void main() {
       },
     );
 
-    testWidgets('Floating snackbar with custom width is centered when text direction is rtl', (WidgetTester tester) async {
+    testWidgets('Material3 - Floating snackbar with custom width is centered when text direction is rtl', (WidgetTester tester) async {
       // Regression test for https://github.com/flutter/flutter/issues/140125.
       const double customWidth = 400.0;
       await tester.pumpWidget(
@@ -2631,7 +2631,51 @@ void main() {
       );
 
       await tester.tap(find.text('X'));
-      await tester.pump(); // start animation
+      await tester.pump(); // Start animation.
+      await tester.pump(const Duration(milliseconds: 750));
+
+      final Finder materialFinder = find.descendant(
+        of: find.byType(SnackBar),
+        matching: find.byType(Material),
+      );
+      final Offset snackBarBottomLeft = tester.getBottomLeft(materialFinder);
+      final Offset snackBarBottomRight = tester.getBottomRight(materialFinder);
+      expect(snackBarBottomLeft.dx, (800 - customWidth) / 2); // Device width is 800.
+      expect(snackBarBottomRight.dx, (800 + customWidth) / 2); // Device width is 800.
+    });
+
+    testWidgets('Material2 - Floating snackbar with custom width is centered when text direction is rtl', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/147838.
+      const double customWidth = 400.0;
+      await tester.pumpWidget(
+        MaterialApp(
+          theme: ThemeData(useMaterial3: false),
+          home: Directionality(
+            textDirection: TextDirection.rtl,
+            child: Scaffold(
+              body: Builder(
+                builder: (BuildContext context) {
+                  return GestureDetector(
+                    onTap: () {
+                      ScaffoldMessenger.of(context).showSnackBar(
+                        const SnackBar(
+                          behavior: SnackBarBehavior.floating,
+                          width: customWidth,
+                          content: Text('Feeling super snackish'),
+                        ),
+                      );
+                    },
+                    child: const Text('X'),
+                  );
+                },
+              ),
+            ),
+          ),
+        ),
+      );
+
+      await tester.tap(find.text('X'));
+      await tester.pump(); // Start animation.
       await tester.pump(const Duration(milliseconds: 750));
 
       final Finder materialFinder = find.descendant(