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

Inizia individuando l’implementazione esistente di LiveAnnouncer e analizzando la riproduzione StackBlitz collegata. Determina come gli annunci accodati debbano rapportarsi al comportamento attuale a elemento singolo e se l’implementazione accodata lo sostituisca o sia selezionabile. Il lavoro è completato quando i messaggi consecutivi e ripetuti vengono annunciati nell’ordine e la decisione progettuale è stata risolta.

Scritto dal modello di indicizzazione a partire dal testo della issue.