Update Android predictive back and related docs

[parlough]

The following docs and potential others need updates for the current state of Android predictive back support and related API changes.

• Add the predictive-back gesture needs updates for the current status of support in Flutter and Android. It also needs additional documentation.
• Android predictive back (migration guide) needs updates around the change from PopScope.onPopInvoked to PopScope.onPopInvokedWithResult. It also should rely on the primary predictive back doc rather than having its own "Supporting predictive back" section that gets out of date.
• Navigation and routing (index) needs updates relating to its docs, at least those about WillPopScope.
• Add a Flutter Fragment to an Android app needs changes due to APIs related to handling back navigation (shouldAutomaticallyHandleOnBackPressed).
• Potentially: Add a Flutter view to an Android app in a similar way to the fragment page.
• Potentially: Automatic platform adaptations
• Potentially: Animate a page route transition

Various references:

• https://developer.android.com/guide/navigation/custom-back/predictive-back-gesture