fix(router): adjust setting navigationTransition when a new navigatio…

[jasonaden]

…n cancels an existing one

Prior to this change, if a navigation was ongoing and a new one came in, the router could get into a state where router.currentNavigation was null even though a navigation was executing. This change moves where we set the currentNavigation value so it's inside a switchMap. This solves the problem because the finally on the switchMap had been setting currentNavigation to null but the new currentNavigation value would have already been set. Essentially this was a timing problem and is resolved with this change.

Fixes #29389 #29590

[jasonaden]

Presubmit

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