[web] move AccessibilityAnnouncements into SemanticsOwner

[yjbanov]

Move AccessibilityAnnouncements into SemanticsOwner, effectively making it a singleton (because SemanticsOwner is).

Fixes flutter/flutter#139272

[ditman]

LGTM! Thanks for cleaning this up!

[ditman]

The DOM manager seems to be complaining (maybe others):

00:23 +654 ~26 -1: view_embedder/dom_manager_test.dart: DomManager DOM tree looks right [E]                                                                                                            
  Expected: <3>
    Actual: <2>
  
  org-dartlang-sdk:///lib/_internal/js_runtime/lib/js_helper.dart 1190:19                           Object.wrapException
  org-dartlang-sdk:///lib/_internal/js_runtime/lib/js_helper.dart 1231:28                           Object.throwExpression
  ..\home\chrome-bot\.pub-cache\hosted\pub.dev\matcher-0.12.16\lib\src\expect\expect.dart 149:31    Object.fail
  ..\home\chrome-bot\.pub-cache\hosted\pub.dev\matcher-0.12.16\lib\src\expect\expect.dart 144:3     Object._expect
  ..\home\chrome-bot\.pub-cache\hosted\pub.dev\matcher-0.12.16\lib\src\expect\expect.dart 56:3      Object.expect
  view_embedder\dom_manager_test.dart 41:7                                                          doTests.<fn>.<fn>
  ..\home\chrome-bot\.pub-cache\hosted\pub.dev\test_api-0.6.1\lib\src\backend\declarer.dart 215:15  <fn>
  org-dartlang-sdk:///lib/_internal/js_runtime/lib/async_patch.dart 303:19                          _wrapJsFunctionForAsync.closure.$protected
  org-dartlang-sdk:///lib/_internal/js_runtime/lib/async_patch.dart 328:23                          _wrapJsFunctionForAsync.<fn>
  org-dartlang-sdk:///lib/_internal/js_runtime/lib/async_patch.dart 279:19                          _awaitOnObject.<fn>
  

To run this test again: C:\b\s\w\ir\cache\builder\src\flutter\prebuilts\windows-x64\dart-sdk\bin\dart test view_embedder/dom_manager_test.dart -p chrome --plain-name "DomManager DOM tree looks right"

[yjbanov]

Fixed.