feat(COMPONENT): allow multiple instances of AriaDescriber

[yann-armelin-1a]

Feature Description

Allow to have several AriaDescriber containers on a same page.
Today the container of AriaDescriber is necessarily appended to body, with an hardcoded id, and any pre-existing container is removed.

I see At least 3 different options to achieve this:

1. Do something similar to what's done in LiveAnnouncer: use an Injection token to inject the container.

2. Make the following constant customizable: MESSAGES_CONTAINER_ID . So that each instance of AriaDescriber can have his own container.

3. Do something similar to OverlayContainer so that the container can be moved somewhere else on the page, and stop relying on the ID the remove it

In all cases, the following const should be customizable: CDK_DESCRIBEDBY_ID_PREFIX, otherwise elements with duplicated ID will be created on the page.

Use Case

In the context of micro frontends, we may end up with 2 Angular Material instances on the same page.

Today, a second instance of AriaDescriber would remove the container of the first one, making it ineffective.

[crisbeto]

Looking through it, I think that it should mostly work already. The messagesContainer is a global variable which means that even if you ended up with multiple instances of the AriaDescriber, they'll all still use the same container.

That being said, I think that this will break down if the an AriaDescriber is destroyed while another one is still live, because we remove the global container in ngOnDestroy. It'll also break down one AriaDescriber is initialized later than another one, because the descriptions of the former will be removed by the latter. I think that these are somewhat edge cases, but they can definitely happen.

I think the way to go is to do something similar to the OverlayContainer. I'll look into it later today.