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Creates new class ContentSuggestionsSchedulerAppAgent which brings
together all the Content Suggestion service notifications code.

Introduces AppStateAgents (app state associated objects).
Adds a new AppStateObserver method, sceneDidConnect:, that allows for
unified logic between iOS 12 and 13.

Bug: 1072366,1094916
Change-Id: Idd520e71668e6fc230b73eb3485085bf423b39bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2414223
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809925}