Fix crash if InsetObserver can no longer be created

If an inset observer cannot be created for
ActivityKeyboardVisibilityDelegate because the activity is being
finished then return an empty ObservableSupplier instead.

Fixed: 346091025
Change-Id: I9c5d421f8f48ed0e98cb2083beef6b558ecf29df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5616610


Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1313049}