TF-2280 Call BackgroundIsolateBinaryMessenger.ensureInitialized on cross-platform

Signed-off-by: dab246 <tdvu@linagora.com>
(cherry picked from commit c2bcbd604f3a6ea96288253efc527be51ad6d178)

lib/features/base/isolate/background_isolate_binary_messenger/background_isolate_binary_messenger.dart

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+
+export 'background_isolate_binary_messenger_web.dart'
+  if (dart.library.html) 'background_isolate_binary_messenger_web.dart' // Browser
+  if (dart.library.io) 'background_isolate_binary_messenger_mobile.dart'; // VM

lib/features/base/isolate/background_isolate_binary_messenger/background_isolate_binary_messenger_mobile.dart

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+
+export 'package:flutter/services.dart';

lib/features/base/isolate/background_isolate_binary_messenger/background_isolate_binary_messenger_web.dart

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+
+class RootIsolateToken {
+  static final RootIsolateToken? instance = () {}();
+}
+
+class BackgroundIsolateBinaryMessenger {
+  static void ensureInitialized(RootIsolateToken token) {}
+}