[ios] Set startupHadExternalIntent when called from Shortcuts app

When the app is started via Shortcuts app (or similar intents), it
is invoked with UISceneConnectionOptions.userActivities which is a
non-empty container.

Update scene:willConntectToSession:options: to correctly set the
-startupHadExternalIntent property to YES in that case.

This mirror the logic in -[SceneController handleExternalIntents]
making it sure that workflow that changes when an external intent
is processed is in sync with said intent processing.

    Also use a local variable to store `self.sceneState` to avoid
    repeatedly accessing it through the property (which can cause
    refcount churn when using ARC).

Bug: 40769058
Change-Id: Idd577476156ecb5058898263a4b5ec514010491d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425053


Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283799}