live-announcer: allow custom live element

[devversion]

As mentioned in the originally PR, we'd like to have a possibility to use a custom live element.

There will be two possibilities:

• Dependency Injection (OpaqueToken) for the Element
• Public Service Method, to overwrite the Live Element.

I'm up to implemented that.

[jelbourn]

I think the OpaqueToken approach is good.

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