fix(live-announcer): duplicate live element when coming in from the server

[crisbeto]

Fixes the case where the user might get multiple live announcer elements, if they're coming in from a server-side-rendered page. Along the same lines as #11940.

[devversion]

Code-wise LGTM. Wouldn't it be more reasonable to just not generate the live element on server-side?

Since the live-announcer depends on a actual DOM that can change it's state, pre-rendered live-elements don't really make any sense if I don't miss anything.

[crisbeto]

I suppose, but then we're going down different code paths where in one case we have an element and in the other we don't. This means that in the latter we'd also have to have extra null checks everywhere or keep the element only in memory.

[devversion]

I think the overlay is a bit different though. The overlay is most likely something that's visually present, and would be part of the pre-rendered view.

I see the point of having extra null checks, but I kind of think that this would be easier and more performant than just iterating through existing elements and deleting them. I don't really care that much. Just wanted to propose an alternative approach.

[jelbourn]

LGTM

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