Switch gr-app-element to a router model subscription
Switch gr-app-element to a router model subscription

We want to remove the element's dependency on `params` step by step.
It should not require gr-router to set the `params` property. Instead
we are listening to models.

Release-Notes: skip
Google-Bug-Id: b/244279450
Change-Id: Ia1afb274e7a26325598aadb8748084270558bde0

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