Migrate platform_channels to Material 3 (#1623)

* Migrate platform_channels to Material 3

* fix tests

---------

Co-authored-by: Brett Morgan <brettmorgan@google.com>

platform_channels/lib/main.dart

@@ -25,6 +25,7 @@ class PlatformChannelSample extends StatelessWidget {
         snackBarTheme: SnackBarThemeData(
           backgroundColor: Colors.blue[500],
         ),
+        useMaterial3: true,
       ),
       routerConfig: router(),
     );