Queue messages sent to LiveAnnouncer

#### Feature Description

LiveAnnouncer should be able to queue multiple messages and announce them one by one instead of announcing only the lates message.

#### Use Case

See https://stackblitz.com/edit/angular-b4jomd, it has a button which announces two messages "Table Foo was updated" and "Table Bar was updated" on click. It illustrates two use cases:

1) Simultaneous announcements. Current implementation of LiveAnnouncer uses a single DOM element for the announcement, so second message overrides the first one and screen reader users hear only the second message.

2) Consequent announcement of the same message. Current implementation will announce only distinct messages, i.e. clicking the button multiple times will produce only one message for the screen reader. That's not always convenient. For example, if you have a table on the page and you do some polling which updates the table 5 times, we need to be able to announce every of those updates so that screen reader users know what's happening on the page.

#### Implementation

We already have an implementation of a queued LiveAnnouncer which was used in GCP for the last 5 months, it alowed to fix a lot of accessibility problems on pages with heavy UIs. I will convert it to a PR here, but I'd like to have some discussion first. At the very least I'd like to know whether it should replace the existing LiveAnnouncer implementation or we should allow users to switch between two implementations

まず既存の LiveAnnouncer 実装を見つけ、リンクされている StackBlitz の再現を確認します。キューに入れられたアナウンスを現在の単一要素の動作とどのように関連付けるべきか、またキューを使用する実装がそれを置き換えるのか、選択可能なのかを判断します。連続したメッセージと繰り返されたメッセージが順番どおりにアナウンスされ、設計上の決定が解決されれば完了です。

索引モデルが issue の本文から書いたものです。