Issue #18519: cdk overlay container removed fix

[Tanemahuta]

So, this is my fix. It works quite well.
The problem was:

• another angular app (e.g. sideloaded) or other JS code has removed the 'cdk-overlay-container'
• since all was cached (also the OverlayRefs in the components), the container was not re-created/re-appended to the DOM
  My solution sketch:
• On the OverlayContainer component:
• additional check if the HTML element for the overlay has a parentNode (= within the DOM)
• if the container was removed, it will be re-added (and the others will be removed)
• On the Overlay service:
• wrap the PortalOutlet adding a call to the OverlayContainer component which calls the getContainer method and ensures that the HTML element is where it is supposed to be.

Et voilà: it works.

Sorry that I did not include a test, but you should get the idea.

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[Tanemahuta]

@googlebot I fixed it.

[crisbeto]

Have you tested out the issue that you described with the changes from #17006 (released in 9.0.0)? It should prevent the container from being removed unnecessarily.

[devversion]

@Tanemahuta Thanks for the proposed fix. Very much appreciated.

I'm closing this PR though as it looks the fix from @crisbeto already resolved this. See: https://stackblitz.com/edit/comp-18519?file=src%2Fapp%2Felement%2Felement.module.ts.

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