add check for announcement support per platform (flutter#166099)

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Helps unblock flutter#165531 and is part
of flutter#165510

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Author: ash2moon

packages/flutter/lib/src/semantics/semantics_service.dart

@@ -7,6 +7,7 @@ library;
 
 import 'dart:ui' show TextDirection;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart' show SystemChannels;
 
 import 'semantics_event.dart' show AnnounceSemanticsEvent, Assertiveness, TooltipSemanticsEvent;
@@ -33,6 +34,9 @@ abstract final class SemanticsService {
   /// Currently, this is only supported by the web engine and has no effect on
   /// other platforms. The default mode is [Assertiveness.polite].
   ///
+  /// Not all platforms support announcements. Check to see if
+  /// [isAnnounceSupported] before calling this method.
+  ///
   /// ### Android
   /// Android has [deprecated announcement events][1] due to its disruptive
   /// behavior with TalkBack forcing it to clear its speech queue and speak the
@@ -62,4 +66,12 @@ abstract final class SemanticsService {
     final TooltipSemanticsEvent event = TooltipSemanticsEvent(message);
     await SystemChannels.accessibility.send(event.toMap());
   }
+
+  /// Checks if announce is supported on the given platform.
+  ///
+  /// On Android the announce method is deprecated, therefore will return false.
+  /// On other platforms, this will return true.
+  static bool isAnnounceSupported() {
+    return defaultTargetPlatform != TargetPlatform.android;
+  }
 }

packages/flutter/test/semantics/semantics_service_test.dart

@@ -2,8 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/semantics.dart';
-import 'package:flutter/services.dart' show SystemChannels;
+import 'package:flutter/services.dart' show SystemChannels, TargetPlatform;
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -44,4 +45,12 @@ void main() {
       ]),
     );
   });
+
+  for (final TargetPlatform platform in TargetPlatform.values) {
+    test('Announce not supported on Android. (platform=$platform)', () {
+      debugDefaultTargetPlatformOverride = platform;
+      expect(SemanticsService.isAnnounceSupported(), platform != TargetPlatform.android);
+      debugDefaultTargetPlatformOverride = null;
+    });
+  }
 }