feat(server): allow ability to specify breakpoints to activate on server

[CaerusKaru]

On the server, there is no conception of "browser window," because Angular's
browser implementation doesn't implement it. This means MatchMedia breakpoint activations will not work on the server.

Layout SSR will generate CSS Stylesheets for all known breakpoints. And for the static, delivered-first HTML pages the directives are replaced with their associated, generated CSS classnames.

But for programmatic usages of MediaObserver, we don't have a fallback mechanism to announce breakpoint activations. This PR allows a user to specify which activations they want to trigger via the MediaObserver for programmatic view listeners.

This can be done using the following configuration:

FlexLayoutModule.withConfig({serverBreakpoints: ['xs', 'lt-md']})

Fixes #991

[ThomasBurleson]

For clarity, please elaborate on problem and solution in PR body.

[CaerusKaru]

@ThomasBurleson added

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