fix(cdk/listbox): improve SSR compatibility by adding an _isBrowser check before calling _setPreviousActiveOptionAsActiveOptionOnWindowBlur

[james-albanese]

This PR addresses the ReferenceError: window is not defined error that gets thrown when the CDK Listbox is rendered on the server.

The _setPreviousActiveOptionAsActiveOptionOnWindowBlur method in the CdkListbox class uses RxJS' fromEvent with window as its target. This is problematic when the application is being rendered on the server, since _setPreviousActiveOptionAsActiveOptionOnWindowBlur is called in the constructor.

The solution is to add a simple _isBrowser property that leverages the Platform API and a check in the constructor before calling _setPreviousActiveOptionAsActiveOptionOnWindowBlur.

[crisbeto]

LGTM

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